LDP SpecificationRFC 5036

Network Working Group L. Andersson, Ed.
Request for Comments: 5036 Acreo AB
Obsoletes: 3036 I. Minei, Ed.
Category: Standards Track Juniper Networks
B. Thomas, Ed.
Cisco Systems, Inc.
October 2007
LDP Specification
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
The architecture for Multiprotocol Label Switching (MPLS) is
described in RFC 3031. A fundamental concept in MPLS is that two
Label Switching Routers (LSRs) must agree on the meaning of the
labels used to forward traffic between and through them. This common
understanding is achieved by using a set of procedures, called a
label distribution protocol, by which one LSR informs another of
label bindings it has made. This document defines a set of such
procedures called LDP (for Label Distribution Protocol) by which LSRs
distribute labels to support MPLS forwarding along normally routed
paths.
Table of Contents
1. LDP Overview ....................................................51.1. LDP Peers ..................................................61.2. LDP Message Exchange .......................................61.3. LDP Message Structure ......................................71.4. LDP Error Handling .........................................71.5. LDP Extensibility and Future Compatibility .................71.6. Specification Language .....................................72. LDP Operation ...................................................82.1. FECs .......................................................82.2. Label Spaces, Identifiers, Sessions, and Transport .........92.2.1. Label Spaces ........................................92.2.2. LDP Identifiers .....................................92.2.3. LDP Sessions .......................................102.2.4. LDP Transport ......................................10Andersson, et al. Standards Track [Page 1]

RFC 5036 LDP Specification October 20071. LDP Overview
The MPLS architecture [RFC3031] defines a label distribution protocol
as a set of procedures by which one Label Switched Router (LSR)
informs another of the meaning of labels used to forward traffic
between and through them.
The MPLS architecture does not assume a single label distribution
protocol. In fact, a number of different label distribution
protocols are being standardized. Existing protocols have been
extended so that label distribution can be piggybacked on them. New
protocols have also been defined for the explicit purpose of
distributing labels. The MPLS architecture discusses some of the
considerations when choosing a label distribution protocol for use in
particular MPLS applications such as Traffic Engineering [RFC2702].
The Label Distribution Protocol (LDP) is a protocol defined for
distributing labels. It was originally published as RFC 3036 in
January 2001. It was produced by the MPLS Working Group of the IETF
and was jointly authored by Loa Andersson, Paul Doolan, Nancy
Feldman, Andre Fredette, and Bob Thomas.
LDP is a protocol defined for distributing labels. It is the set of
procedures and messages by which Label Switched Routers (LSRs)
establish Label Switched Paths (LSPs) through a network by mapping
network-layer routing information directly to data-link layer
switched paths. These LSPs may have an endpoint at a directly
attached neighbor (comparable to IP hop-by-hop forwarding), or may
have an endpoint at a network egress node, enabling switching via all
intermediary nodes.
LDP associates a Forwarding Equivalence Class (FEC) [RFC3031] with
each LSP it creates. The FEC associated with an LSP specifies which
packets are "mapped" to that LSP. LSPs are extended through a
network as each LSR "splices" incoming labels for a FEC to the
outgoing label assigned to the next hop for the given FEC.
More information about the applicability of LDP can be found in
[RFC3037].
This document assumes (but does not require) familiarity with the
MPLS architecture [RFC3031]. Note that [RFC3031] includes a glossary
of MPLS terminology, such as ingress, label switched path, etc.
Andersson, et al. Standards Track [Page 5]

RFC 5036 LDP Specification October 20071.1. LDP Peers
Two LSRs that use LDP to exchange label/FEC mapping information are
known as "LDP Peers" with respect to that information, and we speak
of there being an "LDP Session" between them. A single LDP session
allows each peer to learn the other's label mappings; i.e., the
protocol is bidirectional.
1.2. LDP Message Exchange
There are four categories of LDP messages:
1. Discovery messages, used to announce and maintain the presence
of an LSR in a network.
2. Session messages, used to establish, maintain, and terminate
sessions between LDP peers.
3. Advertisement messages, used to create, change, and delete
label mappings for FECs.
4. Notification messages, used to provide advisory information and
to signal error information.
Discovery messages provide a mechanism whereby LSRs indicate their
presence in a network by sending a Hello message periodically. This
is transmitted as a UDP packet to the LDP port at the 'all routers on
this subnet' group multicast address. When an LSR chooses to
establish a session with another LSR learned via the Hello message,
it uses the LDP initialization procedure over TCP transport. Upon
successful completion of the initialization procedure, the two LSRs
are LDP peers, and may exchange advertisement messages.
When to request a label or advertise a label mapping to a peer is
largely a local decision made by an LSR. In general, the LSR
requests a label mapping from a neighboring LSR when it needs one,
and advertises a label mapping to a neighboring LSR when it wishes
the neighbor to use a label.
Correct operation of LDP requires reliable and in-order delivery of
messages. To satisfy these requirements, LDP uses the TCP transport
for Session, Advertisement, and Notification messages, i.e., for
everything but the UDP-based discovery mechanism.
Andersson, et al. Standards Track [Page 6]

RFC 5036 LDP Specification October 20071.3. LDP Message Structure
All LDP messages have a common structure that uses a Type-Length-
Value (TLV) encoding scheme; see Section "Type-Length-Value
Encoding". The Value part of a TLV-encoded object, or TLV for short,
may itself contain one or more TLVs.
1.4. LDP Error Handling
LDP errors and other events of interest are signaled to an LDP peer
by Notification messages.
There are two kinds of LDP Notification messages:
1. Error Notifications, used to signal fatal errors. If an LSR
receives an Error Notification from a peer for an LDP session,
it terminates the LDP session by closing the TCP transport
connection for the session and discarding all label mappings
learned via the session.
2. Advisory Notifications, used to pass on LSR information about
the LDP session or the status of some previous message received
from the peer.
1.5. LDP Extensibility and Future Compatibility
Functionality may be added to LDP in the future. It is likely that
future functionality will utilize new messages and object types
(TLVs). It may be desirable to employ such new messages and TLVs
within a network using older implementations that do not recognize
them. While it is not possible to make every future enhancement
backwards compatible, some prior planning can ease the introduction
of new capabilities. This specification defines rules for handling
unknown message types and unknown TLVs for this purpose.
1.6. Specification Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Andersson, et al. Standards Track [Page 7]

RFC 5036 LDP Specification October 20072. LDP Operation2.1. FECs
It is necessary to precisely specify which packets may be mapped to
each LSP. This is done by providing a FEC specification for each
LSP. The FEC identifies the set of IP packets that may be mapped to
that LSP.
Each FEC is specified as a set of one or more FEC elements. Each FEC
element identifies a set of packets that may be mapped to the
corresponding LSP. When an LSP is shared by multiple FEC elements,
that LSP is terminated at (or before) the node where the FEC elements
can no longer share the same path.
This specification defines a single type of FEC element, the "Address
Prefix FEC element". This element is an address prefix of any length
from 0 to a full address, inclusive.
Additional FEC elements may be defined, as needed, by other
specifications.
In the remainder of this section, we give the rules to be used for
mapping packets to LSPs that have been set up using an Address Prefix
FEC element.
We say that a particular address "matches" a particular address
prefix if and only if that address begins with that prefix. We also
say that a particular packet matches a particular LSP if and only if
that LSP has an Address Prefix FEC element that matches the packet's
destination address. With respect to a particular packet and a
particular LSP, we refer to any Address Prefix FEC element that
matches the packet as the "matching prefix".
The procedure for mapping a particular packet to a particular LSP
uses the following rules. Each rule is applied in turn until the
packet can be mapped to an LSP.
- If a packet matches exactly one LSP, the packet is mapped to
that LSP.
- If a packet matches multiple LSPs, it is mapped to the LSP
whose matching prefix is the longest. If there is no one LSP
whose matching prefix is longest, the packet is mapped to one
from the set of LSPs whose matching prefix is longer than the
others. The procedure for selecting one of those LSPs is
beyond the scope of this document.
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RFC 5036 LDP Specification October 2007
- If it is known that a packet must traverse a particular egress
router, and there is an LSP that has an Address Prefix FEC
element that is a /32 address of that router, then the packet
is mapped to that LSP. The procedure for obtaining this
knowledge is beyond the scope of this document.
The procedure for determining that a packet must traverse a
particular egress router is beyond the scope of this document. (As
an example, if one is running a link state routing algorithm, it may
be possible to obtain this information from the link state data base.
As another example, if one is running BGP, it may be possible to
obtain this information from the BGP next hop attribute of the
packet's route.)
2.2. Label Spaces, Identifiers, Sessions, and Transport2.2.1. Label Spaces
The notion of "label space" is useful for discussing the assignment
and distribution of labels. There are two types of label spaces:
- Per interface label space. Interface-specific incoming labels
are used for interfaces that use interface resources for
labels. An example of such an interface is a label-controlled
ATM interface that uses VCIs (Virtual Channel Identifiers) as
labels, or a Frame Relay interface that uses DLCIs (Data Link
Connection Identifiers) as labels.
Note that the use of a per interface label space only makes
sense when the LDP peers are "directly connected" over an
interface, and the label is only going to be used for traffic
sent over that interface.
- Per platform label space. Platform-wide incoming labels are
used for interfaces that can share the same labels.
2.2.2. LDP Identifiers
An LDP Identifier is a six octet quantity used to identify an LSR
label space. The first four octets identify the LSR and must be a
globally unique value, such as a 32-bit router Id assigned to the
LSR. The last two octets identify a specific label space within the
LSR. The last two octets of LDP Identifiers for platform-wide label
spaces are always both zero. This document uses the following print
representation for LDP Identifiers:
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RFC 5036 LDP Specification October 2007
<LSR Id> : <label space id>
e.g., lsr171:0, lsr19:2.
Note that an LSR that manages and advertises multiple label spaces
uses a different LDP Identifier for each such label space.
A situation where an LSR would need to advertise more than one label
space to a peer and hence use more than one LDP Identifier occurs
when the LSR has two links to the peer and both are ATM (and use per
interface labels). Another situation would be where the LSR had two
links to the peer, one of which is ethernet (and uses per platform
labels) and the other of which is ATM.
2.2.3. LDP Sessions
LDP sessions exist between LSRs to support label exchange between
them.
When an LSR uses LDP to advertise more than one label space to
another LSR, it uses a separate LDP session for each label space.
2.2.4. LDP Transport
LDP uses TCP as a reliable transport for sessions.
When multiple LDP sessions are required between two LSRs, there is
one TCP session for each LDP session.
2.3. LDP Sessions between Non-Directly Connected LSRs
LDP sessions between LSRs that are not directly connected at the link
level may be desirable in some situations.
For example, consider a "traffic engineering" application where LSRa
sends traffic matching some criteria via an LSP to non-directly
connected LSRb rather than forwarding the traffic along its normally
routed path.
The path between LSRa and LSRb would include one or more intermediate
LSRs (LSR1,...LSRn). An LDP session between LSRa and LSRb would
enable LSRb to label switch traffic arriving on the LSP from LSRa by
providing LSRb means to advertise labels for this purpose to LSRa.
In this situation, LSRa would apply two labels to traffic it forwards
on the LSP to LSRb: a label learned from LSR1 to forward traffic
along the LSP path from LSRa to LSRb; and a label learned from LSRb
to enable LSRb to label switch traffic arriving on the LSP.
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RFC 5036 LDP Specification October 2007
LSRa first adds the label learned via its LDP session with LSRb to
the packet label stack (either by replacing the label on top of the
packet label stack with it if the packet arrives labeled or by
pushing it if the packet arrives unlabeled). Next, it pushes the
label for the LSP learned from LSR1 onto the label stack.
2.4. LDP Discovery
LDP discovery is a mechanism that enables an LSR to discover
potential LDP peers. Discovery makes it unnecessary to explicitly
configure an LSR's label switching peers.
There are two variants of the discovery mechanism:
- A Basic Discovery mechanism used to discover LSR neighbors that
are directly connected at the link level.
- An Extended Discovery mechanism used to locate LSRs that are
not directly connected at the link level.
2.4.1. Basic Discovery Mechanism
To engage in LDP Basic Discovery on an interface, an LSR periodically
sends LDP Link Hellos out the interface. LDP Link Hellos are sent as
UDP packets addressed to the well-known LDP discovery port for the
"all routers on this subnet" group multicast address.
An LDP Link Hello sent by an LSR carries the LDP Identifier for the
label space the LSR intends to use for the interface and possibly
additional information.
Receipt of an LDP Link Hello on an interface identifies a "Hello
adjacency" with a potential LDP peer reachable at the link level on
the interface as well as the label space the peer intends to use for
the interface.
2.4.2. Extended Discovery Mechanism
LDP sessions between non-directly connected LSRs are supported by LDP
Extended Discovery.
To engage in LDP Extended Discovery, an LSR periodically sends LDP
Targeted Hellos to a specific address. LDP Targeted Hellos are sent
as UDP packets addressed to the well-known LDP discovery port at the
specific address.
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RFC 5036 LDP Specification October 2007
An LDP Targeted Hello sent by an LSR carries the LDP Identifier for
the label space the LSR intends to use and possibly additional
optional information.
Extended Discovery differs from Basic Discovery in the following
ways:
- A Targeted Hello is sent to a specific address rather than to
the "all routers" group multicast address for the outgoing
interface.
- Unlike Basic Discovery, which is symmetric, Extended Discovery
is asymmetric.
One LSR initiates Extended Discovery with another targeted LSR,
and the targeted LSR decides whether to respond to or ignore
the Targeted Hello. A targeted LSR that chooses to respond
does so by periodically sending Targeted Hellos to the
initiating LSR.
Receipt of an LDP Targeted Hello identifies a "Hello adjacency" with
a potential LDP peer reachable at the network level and the label
space the peer intends to use.
2.5. Establishing and Maintaining LDP Sessions2.5.1. LDP Session Establishment
The exchange of LDP Discovery Hellos between two LSRs triggers LDP
session establishment. Session establishment is a two step process:
- Transport connection establishment
- Session initialization
The following describes establishment of an LDP session between LSRs
LSR1 and LSR2 from LSR1's point of view. It assumes the exchange of
Hellos specifying label space LSR1:a for LSR1 and label space LSR2:b
for LSR2.
2.5.2. Transport Connection Establishment
The exchange of Hellos results in the creation of a Hello adjacency
at LSR1 that serves to bind the link (L) and the label spaces LSR1:a
and LSR2:b.
1. If LSR1 does not already have an LDP session for the exchange
of label spaces LSR1:a and LSR2:b, it attempts to open a TCP
connection for a new LDP session with LSR2.
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RFC 5036 LDP Specification October 2007
LSR1 determines the transport addresses to be used at its end
(A1) and LSR2's end (A2) of the LDP TCP connection. Address A1
is determined as follows:
a. If LSR1 uses the Transport Address optional object (TLV) in
Hellos it sends to LSR2 to advertise an address, A1 is the
address LSR1 advertises via the optional object;
b. If LSR1 does not use the Transport Address optional object,
A1 is the source address used in Hellos it sends to LSR2.
Similarly, address A2 is determined as follows:
a. If LSR2 uses the Transport Address optional object, A2 is
the address LSR2 advertises via the optional object;
b. If LSR2 does not use the Transport Address optional object,
A2 is the source address in Hellos received from LSR2.
2. LSR1 determines whether it will play the active or passive role
in session establishment by comparing addresses A1 and A2 as
unsigned integers. If A1 > A2, LSR1 plays the active role;
otherwise, it is passive.
The procedure for comparing A1 and A2 as unsigned integers is:
- If A1 and A2 are not in the same address family, they are
incomparable, and no session can be established.
- Let U1 be the abstract unsigned integer obtained by treating
A1 as a sequence of bytes, where the byte that appears
earliest in the message is the most significant byte of the
integer and the byte that appears latest in the message is
the least significant byte of the integer.
Let U2 be the abstract unsigned integer obtained from A2 in
a similar manner.
- Compare U1 with U2. If U1 > U2, then A1 > A2; if U1 < U2,
then A1 < A2.
3. If LSR1 is active, it attempts to establish the LDP TCP
connection by connecting to the well-known LDP port at address
A2. If LSR1 is passive, it waits for LSR2 to establish the LDP
TCP connection to its well-known LDP port.
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RFC 5036 LDP Specification October 2007
Note that when an LSR sends a Hello, it selects the transport address
for its end of the session connection and uses the Hello to advertise
the address, either explicitly by including it in an optional
Transport Address TLV or implicitly by omitting the TLV and using it
as the Hello source address.
An LSR MUST advertise the same transport address in all Hellos that
advertise the same label space. This requirement ensures that two
LSRs linked by multiple Hello adjacencies using the same label spaces
play the same connection establishment role for each adjacency.
2.5.3. Session Initialization
After LSR1 and LSR2 establish a transport connection, they negotiate
session parameters by exchanging LDP Initialization messages. The
parameters negotiated include LDP protocol version, label
distribution method, timer values, VPI/VCI (Virtual Path Identifier /
Virtual Channel Identifier) ranges for label controlled ATM, DLCI
(Data Link Connection Identifier) ranges for label controlled Frame
Relay, etc.
Successful negotiation completes establishment of an LDP session
between LSR1 and LSR2 for the advertisement of label spaces LSR1:a
and LSR2:b.
The following describes the session initialization from LSR1's point
of view.
After the connection is established, if LSR1 is playing the active
role, it initiates negotiation of session parameters by sending an
Initialization message to LSR2. If LSR1 is passive, it waits for
LSR2 to initiate the parameter negotiation.
In general when there are multiple links between LSR1 and LSR2 and
multiple label spaces to be advertised by each, the passive LSR
cannot know which label space to advertise over a newly established
TCP connection until it receives the LDP Initialization message on
the connection. The Initialization message carries both the LDP
Identifier for the sender's (active LSR's) label space and the LDP
Identifier for the receiver's (passive LSR's) label space.
By waiting for the Initialization message from its peer, the passive
LSR can match the label space to be advertised by the peer (as
determined from the LDP Identifier in the PDU header for the
Initialization message) with a Hello adjacency previously created
when Hellos were exchanged.
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RFC 5036 LDP Specification October 2007
1. When LSR1 plays the passive role:
a. If LSR1 receives an Initialization message, it attempts to
match the LDP Identifier carried by the message PDU with a
Hello adjacency.
b. If there is a matching Hello adjacency, the adjacency
specifies the local label space for the session.
Next LSR1 checks whether the session parameters proposed in
the message are acceptable. If they are, LSR1 replies with
an Initialization message of its own to propose the
parameters it wishes to use and a KeepAlive message to
signal acceptance of LSR2's parameters. If the parameters
are not acceptable, LSR1 responds by sending a Session
Rejected/Parameters Error Notification message and closing
the TCP connection.
c. If LSR1 cannot find a matching Hello adjacency, it sends a
Session Rejected/No Hello Error Notification message and
closes the TCP connection.
d. If LSR1 receives a KeepAlive in response to its
Initialization message, the session is operational from
LSR1's point of view.
e. If LSR1 receives an Error Notification message, LSR2 has
rejected its proposed session and LSR1 closes the TCP
connection.
2. When LSR1 plays the active role:
a. If LSR1 receives an Error Notification message, LSR2 has
rejected its proposed session and LSR1 closes the TCP
connection.
b. If LSR1 receives an Initialization message, it checks
whether the session parameters are acceptable. If so, it
replies with a KeepAlive message. If the session parameters
are unacceptable, LSR1 sends a Session Rejected/Parameters
Error Notification message and closes the connection.
c. If LSR1 receives a KeepAlive message, LSR2 has accepted its
proposed session parameters.
d. When LSR1 has received both an acceptable Initialization
message and a KeepAlive message, the session is operational
from LSR1's point of view.
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RFC 5036 LDP Specification October 2007
Until the LDP session is established, no other messages
except those listed in the procedures above may be
exchanged, and the rules for processing the U-bit in LDP
messages are overridden. If a message other than those
listed in the procedures above is received, a Shutdown msg
MUST be transmitted and the transport connection MUST be
closed.
It is possible for a pair of incompatibly configured LSRs that
disagree on session parameters to engage in an endless sequence of
messages as each NAKs the other's Initialization messages with Error
Notification messages.
An LSR MUST throttle its session setup retry attempts with an
exponential backoff in situations where Initialization messages are
being NAK'd. It is also recommended that an LSR detecting such a
situation take action to notify an operator.
The session establishment setup attempt following a NAK'd
Initialization message MUST be delayed no less than 15 seconds, and
subsequent delays MUST grow to a maximum delay of no less than 2
minutes. The specific session establishment action that must be
delayed is the attempt to open the session transport connection by
the LSR playing the active role.
The throttled sequence of Initialization NAKs is unlikely to cease
until operator intervention reconfigures one of the LSRs. After such
a configuration action, there is no further need to throttle
subsequent session establishment attempts (until their Initialization
messages are NAK'd).
Due to the asymmetric nature of session establishment,
reconfiguration of the passive LSR will go unnoticed by the active
LSR without some further action. Section "Hello Message" describes
an optional mechanism an LSR can use to signal potential LDP peers
that it has been reconfigured.
2.5.4. Initialization State Machine
It is convenient to describe LDP session negotiation behavior in
terms of a state machine. We define the LDP state machine to have
five possible states and present the behavior as a state transition
table and as a state transition diagram. Note that a Shutdown
message is implemented as a Notification message with a Status TLV
indicating a fatal error.
Andersson, et al. Standards Track [Page 16]

RFC 5036 LDP Specification October 20072.5.5. Maintaining Hello Adjacencies
An LDP session with a peer has one or more Hello adjacencies.
An LDP session has multiple Hello adjacencies when a pair of LSRs is
connected by multiple links that share the same label space; for
example, multiple PPP links between a pair of routers. In this
situation, the Hellos an LSR sends on each such link carry the same
LDP Identifier.
LDP includes mechanisms to monitor the necessity of an LDP session
and its Hello adjacencies.
LDP uses the regular receipt of LDP Discovery Hellos to indicate a
peer's intent to use the label space identified by the Hello. An LSR
maintains a hold timer with each Hello adjacency that it restarts
when it receives a Hello that matches the adjacency. If the timer
expires without receipt of a matching Hello from the peer, LDP
concludes that the peer no longer wishes to label switch using that
label space for that link (or target, in the case of Targeted Hellos)
or that the peer has failed. The LSR then deletes the Hello
adjacency. When the last Hello adjacency for an LDP session is
deleted, the LSR terminates the LDP session by sending a Notification
message and closing the transport connection.
2.5.6. Maintaining LDP Sessions
LDP includes mechanisms to monitor the integrity of the LDP session.
LDP uses the regular receipt of LDP PDUs on the session transport
connection to monitor the integrity of the session. An LSR maintains
a KeepAlive Timer for each peer session that it resets whenever it
receives an LDP PDU from the session peer. If the KeepAlive Timer
expires without receipt of an LDP PDU from the peer, the LSR
concludes that the transport connection is bad or that the peer has
failed, and it terminates the LDP session by closing the transport
connection.
After an LDP session has been established, an LSR must arrange that
its peer receive an LDP PDU from it at least every KeepAlive time
period to ensure the peer restarts the session KeepAlive Timer. The
LSR may send any protocol message to meet this requirement. In
circumstances where an LSR has no other information to communicate to
its peer, it sends a KeepAlive message.
An LSR may choose to terminate an LDP session with a peer at any
time. Should it choose to do so, it informs the peer with a Shutdown
message.
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RFC 5036 LDP Specification October 20072.6. Label Distribution and Management
The MPLS architecture [RFC3031] allows an LSR to distribute a FEC
label binding in response to an explicit request from another LSR.
This is known as Downstream On Demand label distribution. It also
allows an LSR to distribute label bindings to LSRs that have not
explicitly requested them. [RFC3031] calls this method of label
distribution Unsolicited Downstream; this document uses the term
Downstream Unsolicited.
Both of these label distribution techniques may be used in the same
network at the same time. However, for any given LDP session, each
LSR must be aware of the label distribution method used by its peer
in order to avoid situations where one peer using Downstream
Unsolicited label distribution assumes its peer is also. See Section
"Downstream on Demand Label Advertisement".
2.6.1. Label Distribution Control Mode
The behavior of the initial setup of LSPs is determined by whether
the LSR is operating with independent or Ordered LSP Control. An LSR
may support both types of control as a configurable option.
2.6.1.1. Independent Label Distribution Control
When using independent LSP control, each LSR may advertise label
mappings to its neighbors at any time it desires. For example, when
operating in independent Downstream on Demand mode, an LSR may answer
requests for label mappings immediately, without waiting for a label
mapping from the next hop. When operating in independent Downstream
Unsolicited mode, an LSR may advertise a label mapping for a FEC to
its neighbors whenever it is prepared to label-switch that FEC.
A consequence of using independent mode is that an upstream label can
be advertised before a downstream label is received.
2.6.1.2. Ordered Label Distribution Control
When using LSP Ordered Control, an LSR may initiate the transmission
of a label mapping only for a FEC for which it has a label mapping
for the FEC next hop, or for which the LSR is the egress. For each
FEC for which the LSR is not the egress and no mapping exists, the
LSR MUST wait until a label from a downstream LSR is received before
mapping the FEC and passing corresponding labels to upstream LSRs.
An LSR may be an egress for some FECs and a non-egress for others.
An LSR may act as an egress LSR, with respect to a particular FEC,
under any of the following conditions:
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RFC 5036 LDP Specification October 2007
1. The FEC refers to the LSR itself (including one of its directly
attached interfaces).
2. The next hop router for the FEC is outside of the Label
Switching Network.
3. FEC elements are reachable by crossing a routing domain
boundary, such as another area for OSPF summary networks, or
another autonomous system for OSPF AS externals and BGP routes
[RFC2328] [RFC4271].
Note that whether an LSR is an egress for a given FEC may change over
time, depending on the state of the network and LSR configuration
settings.
2.6.2. Label Retention Mode
The MPLS architecture [RFC3031] introduces the notion of label
retention mode which specifies whether an LSR maintains a label
binding for a FEC learned from a neighbor that is not its next hop
for the FEC.
2.6.2.1. Conservative Label Retention Mode
In Downstream Unsolicited advertisement mode, label mapping
advertisements for all routes may be received from all peer LSRs.
When using Conservative Label retention, advertised label mappings
are retained only if they will be used to forward packets (i.e., if
they are received from a valid next hop according to routing). If
operating in Downstream on Demand mode, an LSR will request label
mappings only from the next hop LSR according to routing. Since
Downstream on Demand mode is primarily used when label conservation
is desired (e.g., an ATM switch with limited cross connect space), it
is typically used with the Conservative Label retention mode.
The main advantage of the conservative mode is that only the labels
that are required for the forwarding of data are allocated and
maintained. This is particularly important in LSRs where the label
space is inherently limited, such as in an ATM switch. A
disadvantage of the conservative mode is that if routing changes the
next hop for a given destination, a new label must be obtained from
the new next hop before labeled packets can be forwarded.
2.6.2.2. Liberal Label Retention Mode
In Downstream Unsolicited advertisement mode, label mapping
advertisements for all routes may be received from all LDP peers.
When using Liberal Label retention, every label mappings received
Andersson, et al. Standards Track [Page 21]

RFC 5036 LDP Specification October 2007
from a peer LSR is retained regardless of whether the LSR is the next
hop for the advertised mapping. When operating in Downstream on
Demand mode with Liberal Label retention, an LSR might choose to
request label mappings for all known prefixes from all peer LSRs.
Note, however, that Downstream on Demand mode is typically used by
devices such as ATM switch-based LSRs for which the conservative
approach is recommended.
The main advantage of the Liberal Label retention mode is that
reaction to routing changes can be quick because labels already
exist. The main disadvantage of the liberal mode is that unneeded
label mappings are distributed and maintained.
2.6.3. Label Advertisement Mode
Each interface on an LSR is configured to operate in either
Downstream Unsolicited or Downstream on Demand advertisement mode.
LSRs exchange advertisement modes during initialization. The major
difference between Downstream Unsolicited and Downstream on Demand
modes is in which LSR takes responsibility for initiating mapping
requests and mapping advertisements.
2.7. LDP Identifiers and Next Hop Addresses
An LSR maintains learned labels in a Label Information Base (LIB).
When operating in Downstream Unsolicited mode, the LIB entry for an
address prefix associates a collection of (LDP Identifier, label)
pairs with the prefix, one such pair for each peer advertising a
label for the prefix.
When the next hop for a prefix changes, the LSR must retrieve the
label advertised by the new next hop from the LIB for use in
forwarding. To retrieve the label, the LSR must be able to map the
next hop address for the prefix to an LDP Identifier.
Similarly, when the LSR learns a label for a prefix from an LDP peer,
it must be able to determine whether that peer is currently a next
hop for the prefix to determine whether it needs to start using the
newly learned label when forwarding packets that match the prefix.
To make that decision, the LSR must be able to map an LDP Identifier
to the peer's addresses to check whether any are a next hop for the
prefix.
To enable LSRs to map between a peer LDP Identifier and the peer's
addresses, LSRs advertise their addresses using LDP Address and
Withdraw Address messages.
Andersson, et al. Standards Track [Page 22]

RFC 5036 LDP Specification October 2007
An LSR sends an Address message to advertise its addresses to a peer.
An LSR sends a Withdraw Address message to withdraw previously
advertised addresses from a peer.
2.8. Loop Detection
Loop Detection is a configurable option that provides a mechanism for
finding looping LSPs and for preventing Label Request messages from
looping in the presence of non-merge capable LSRs.
The mechanism makes use of Path Vector and Hop Count TLVs carried by
Label Request and Label Mapping messages. It builds on the following
basic properties of these TLVs:
- A Path Vector TLV contains a list of the LSRs that its
containing message has traversed. An LSR is identified in a
Path Vector list by its unique LSR Identifier (Id), which is
the first four octets of its LDP Identifier. When an LSR
propagates a message containing a Path Vector TLV, it adds its
LSR Id to the Path Vector list. An LSR that receives a message
with a Path Vector that contains its LSR Id detects that the
message has traversed a loop. LDP supports the notion of a
maximum allowable Path Vector length; an LSR that detects a
Path Vector has reached the maximum length behaves as if the
containing message has traversed a loop.
- A Hop Count TLV contains a count of the LSRS that the
containing message has traversed. When an LSR propagates a
message containing a Hop Count TLV, it increments the count.
An LSR that detects a Hop Count has reached a configured
maximum value behaves as if the containing message has
traversed a loop. By convention, a count of 0 is interpreted
to mean the hop count is unknown. Incrementing an unknown hop
count value results in an unknown hop count value (0).
The following paragraphs describe LDP Loop Detection procedures. For
these paragraphs, and only these paragraphs, "MUST" is redefined to
mean "MUST if configured for Loop Detection". The paragraphs specify
messages that MUST carry Path Vector and Hop Count TLVs. Note that
the Hop Count TLV and its procedures are used without the Path Vector
TLV in situations when Loop Detection is not configured (see
[RFC3035] and [RFC3034]).
2.8.1. Label Request Message
The use of the Path Vector TLV and Hop Count TLV prevent Label
Request messages from looping in environments that include non-merge
capable LSRs.
Andersson, et al. Standards Track [Page 23]

RFC 5036 LDP Specification October 2007
The rules that govern use of the Hop Count TLV in Label Request
messages by LSR R when Loop Detection is enabled are the following:
- The Label Request message MUST include a Hop Count TLV.
- If R is sending the Label Request because it is a FEC ingress, it
MUST include a Hop Count TLV with hop count value 1.
- If R is sending the Label Request as a result of having received a
Label Request from an upstream LSR, and if the received Label
Request contains a Hop Count TLV, R MUST increment the received
hop count value by 1 and MUST pass the resulting value in a Hop
Count TLV to its next hop along with the Label Request message.
The rules that govern use of the Path Vector TLV in Label Request
messages by LSR R when Loop Detection is enabled are the following:
- If R is sending the Label Request because it is a FEC ingress,
then if R is non-merge capable, it MUST include a Path Vector TLV
of length 1 containing its own LSR Id.
- If R is sending the Label Request as a result of having received a
Label Request from an upstream LSR, then if the received Label
Request contains a Path Vector TLV or if R is non-merge capable:
R MUST add its own LSR Id to the Path Vector, and MUST pass the
resulting Path Vector to its next hop along with the Label
Request message. If the Label Request contains no Path Vector
TLV, R MUST include a Path Vector TLV of length 1 containing
its own LSR Id.
Note that if R receives a Label Request message for a particular FEC,
and R has previously sent a Label Request message for that FEC to its
next hop and has not yet received a reply, and if R intends to merge
the newly received Label Request with the existing outstanding Label
Request, then R does not propagate the Label Request to the next hop.
If R receives a Label Request message from its next hop with a Hop
Count TLV that exceeds the configured maximum value, or with a Path
Vector TLV containing its own LSR Id or which exceeds the maximum
allowable length, then R detects that the Label Request message has
traveled in a loop.
When R detects a loop, it MUST send a Loop Detected Notification
message to the source of the Label Request message and drop the Label
Request message.
Andersson, et al. Standards Track [Page 24]

RFC 5036 LDP Specification October 20072.8.2. Label Mapping Message
The use of the Path Vector TLV and Hop Count TLV in the Label Mapping
message provide a mechanism to find and terminate looping LSPs. When
an LSR receives a Label Mapping message from a next hop, the message
is propagated upstream as specified below until an ingress LSR is
reached or a loop is found.
The rules that govern the use of the Hop Count TLV in Label Mapping
messages sent by an LSR R when Loop Detection is enabled are the
following:
- R MUST include a Hop Count TLV.
- If R is the egress, the hop count value MUST be 1.
- If the Label Mapping message is being sent to propagate a Label
Mapping message received from the next hop to an upstream peer,
the hop count value MUST be determined as follows:
o If R is a member of the edge set of an LSR domain whose LSRs do
not perform 'TTL-decrement' (e.g., an ATM LSR domain or a Frame
Relay LSR domain) and the upstream peer is within that domain,
R MUST reset the hop count to 1 before propagating the message.
o Otherwise, R MUST increment the hop count received from the
next hop before propagating the message.
- If the Label Mapping message is not being sent to propagate a
Label Mapping message, the hop count value MUST be the result of
incrementing R's current knowledge of the hop count learned from
previous Label Mapping messages. Note that this hop count value
will be unknown if R has not received a Label Mapping message from
the next hop.
Any Label Mapping message MAY contain a Path Vector TLV. The rules
that govern the mandatory use of the Path Vector TLV in Label Mapping
messages sent by LSR R when Loop Detection is enabled are the
following:
- If R is the egress, the Label Mapping message need not include a
Path Vector TLV.
- If R is sending the Label Mapping message to propagate a Label
Mapping message received from the next hop to an upstream peer,
then:
Andersson, et al. Standards Track [Page 25]

RFC 5036 LDP Specification October 2007
o If R is merge capable and if R has not previously sent a Label
Mapping message to the upstream peer, then it MUST include a
Path Vector TLV.
o If the received message contains an unknown hop count, then R
MUST include a Path Vector TLV.
o If R has previously sent a Label Mapping message to the
upstream peer, then it MUST include a Path Vector TLV if the
received message reports an LSP hop count increase, a change in
hop count from unknown to known, or a change from known to
unknown.
If the above rules require R include a Path Vector TLV in the Label
Mapping message, R computes it as follows:
o If the received Label Mapping message included a Path Vector,
the Path Vector sent upstream MUST be the result of adding R's
LSR Id to the received Path Vector.
o If the received message had no Path Vector, the Path Vector
sent upstream MUST be a Path Vector of length 1 containing R's
LSR Id.
- If the Label Mapping message is not being sent to propagate a
received message upstream, the Label Mapping message MUST include
a Path Vector of length 1 containing R's LSR Id.
If R receives a Label Mapping message from its next hop with a Hop
Count TLV that exceeds the configured maximum value, or with a
Path Vector TLV containing its own LSR Id or that exceeds the
maximum allowable length, then R detects that the corresponding
LSP contains a loop.
When R detects a loop, it MUST stop using the label for
forwarding, drop the Label Mapping message, and signal Loop
Detected status to the source of the Label Mapping message.
2.8.3. Discussion
If Loop Detection is desired in an MPLS domain, then it should be
turned on in ALL LSRs within that MPLS domain, else Loop Detection
will not operate properly and may result in undetected loops or in
falsely detected loops.
LSRs that are configured for Loop Detection are NOT expected to store
the Path Vectors as part of the LSP state.
Andersson, et al. Standards Track [Page 26]

RFC 5036 LDP Specification October 2007
Note that in a network where only non-merge capable LSRs are present,
Path Vectors are passed downstream from ingress to egress, and are
not passed upstream. Even when merge is supported, Path Vectors need
not be passed upstream along an LSP that is known to reach the
egress. When an LSR experiences a change of next hop, it need pass
Path Vectors upstream only when it cannot tell from the hop count
that the change of next hop does not result in a loop.
In the case of ordered label distribution, Label Mapping messages are
propagated from egress toward ingress, naturally creating the Path
Vector along the way. In the case of independent label distribution,
an LSR may originate a Label Mapping message for a FEC before
receiving a Label Mapping message from its downstream peer for that
FEC. In this case, the subsequent Label Mapping message for the FEC
received from the downstream peer is treated as an update to LSP
attributes, and the Label Mapping message must be propagated
upstream. Thus, it is recommended that Loop Detection be configured
in conjunction with ordered label distribution, to minimize the
number of Label Mapping update messages.
2.9. Authenticity and Integrity of LDP Messages
This section specifies a mechanism to protect against the
introduction of spoofed TCP segments into LDP session connection
streams. The use of this mechanism MUST be supported as a
configurable option.
The mechanism is based on use of the TCP MD5 Signature Option
specified in [RFC2385] for use by BGP [RFC4271]. See [RFC1321] for a
specification of the MD5 hash function. From a standards maturity
point of view, the current document relates to [RFC2385] the same way
as [RFC4271] relates to [RFC2385]. This is explained in [RFC4278].
2.9.1. TCP MD5 Signature Option
The following quotes from [RFC2385] outline the security properties
achieved by using the TCP MD5 Signature Option and summarize its
operation:
"IESG Note
This document describes current existing practice for securing
BGP against certain simple attacks. It is understood to have
security weaknesses against concerted attacks."
Andersson, et al. Standards Track [Page 27]

RFC 5036 LDP Specification October 2007
"Abstract
This memo describes a TCP extension to enhance security for
BGP. It defines a new TCP option for carrying an MD5 [RFC1321]
digest in a TCP segment. This digest acts like a signature for
that segment, incorporating information known only to the
connection end points. Since BGP uses TCP as its transport,
using this option in the way described in this paper
significantly reduces the danger from certain security attacks
on BGP."
"Introduction
The primary motivation for this option is to allow BGP to
protect itself against the introduction of spoofed TCP segments
into the connection stream. Of particular concern are TCP
resets.
To spoof a connection using the scheme described in this paper,
an attacker would not only have to guess TCP sequence numbers,
but would also have had to obtain the password included in the
MD5 digest. This password never appears in the connection
stream, and the actual form of the password is up to the
application. It could even change during the lifetime of a
particular connection so long as this change was synchronized
on both ends (although retransmission can become problematical
in some TCP implementations with changing passwords).
Finally, there is no negotiation for the use of this option in
a connection, rather it is purely a matter of site policy
whether or not its connections use the option."
"MD5 as a Hashing Algorithm
Since this memo was first issued (under a different title), the
MD5 algorithm has been found to be vulnerable to collision
search attacks [Dobb], and is considered by some to be
insufficiently strong for this type of application.
This memo still specifies the MD5 algorithm, however, since the
option has already been deployed operationally, and there was
no "algorithm type" field defined to allow an upgrade using the
same option number. The original document did not specify a
type field since this would require at least one more byte, and
it was felt at the time that taking 19 bytes for the complete
option (which would probably be padded to 20 bytes in TCP
implementations) would be too much of a waste of the already
limited option space.
Andersson, et al. Standards Track [Page 28]

RFC 5036 LDP Specification October 2007
This does not prevent the deployment of another similar option
which uses another hashing algorithm (like SHA-1). Also, if
most implementations pad the 18 byte option as defined to 20
bytes anyway, it would be just as well to define a new option
which contains an algorithm type field.
This would need to be addressed in another document, however."
End of quotes from [RFC2385].
2.9.2. LDP Use of TCP MD5 Signature Option
LDP uses the TCP MD5 Signature Option as follows:
- Use of the MD5 Signature Option for LDP TCP connections is a
configurable LSR option.
- An LSR that uses the MD5 Signature Option is configured with a
password (shared secret) for each potential LDP peer.
- The LSR applies the MD5 algorithm as specified in [RFC2385] to
compute the MD5 digest for a TCP segment to be sent to a peer.
This computation makes use of the peer password as well as the
TCP segment.
- When the LSR receives a TCP segment with an MD5 digest, it
validates the segment by calculating the MD5 digest (using its
own record of the password) and compares the computed digest
with the received digest. If the comparison fails, the segment
is dropped without any response to the sender.
- The LSR ignores LDP Hellos from any LSR for which a password
has not been configured. This ensures that the LSR establishes
LDP TCP connections only with LSRs for which a password has
been configured.
2.10. Label Distribution for Explicitly Routed LSPs
Traffic Engineering [RFC2702] is expected to be an important MPLS
application. MPLS support for Traffic Engineering uses explicitly
routed LSPs, which need not follow normally-routed (hop-by-hop) paths
as determined by destination-based routing protocols. CR-LDP [CRLDP]
defines extensions to LDP to use LDP to set up explicitly routed
LSPs.
Andersson, et al. Standards Track [Page 29]

RFC 5036 LDP Specification October 20073. Protocol Specification
Previous sections that describe LDP operation have discussed
scenarios that involve the exchange of messages among LDP peers.
This section specifies the message encodings and procedures for
processing the messages.
LDP message exchanges are accomplished by sending LDP protocol data
units (PDUs) over LDP session TCP connections.
Each LDP PDU can carry one or more LDP messages. Note that the
messages in an LDP PDU need not be related to one another. For
example, a single PDU could carry a message advertising FEC-label
bindings for several FECs, another message requesting label bindings
for several other FECs, and a third Notification message signaling
some event.
3.1. LDP PDUs
Each LDP PDU is an LDP header followed by one or more LDP messages.
The LDP header is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version | PDU Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LDP Identifier |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version
Two octet unsigned integer containing the version number of the
protocol. This version of the specification specifies LDP
protocol version 1.
PDU Length
Two octet integer specifying the total length of this PDU in
octets, excluding the Version and PDU Length fields.
The maximum allowable PDU Length is negotiable when an LDP session
is initialized. Prior to completion of the negotiation, the
maximum allowable length is 4096 bytes.
Andersson, et al. Standards Track [Page 30]

RFC 5036 LDP Specification October 2007
LDP Identifier
Six octet field that uniquely identifies the label space of the
sending LSR for which this PDU applies. The first four octets
identify the LSR and MUST be a globally unique value. It SHOULD
be a 32-bit router Id assigned to the LSR and also used to
identify it in Loop Detection Path Vectors. The last two octets
identify a label space within the LSR. For a platform-wide label
space, these SHOULD both be zero.
Note that there is no alignment requirement for the first octet of an
LDP PDU.
3.2. LDP Procedures
LDP defines messages, TLVs, and procedures in the following areas:
- Peer discovery
- Session management
- Label distribution
- Notification of errors and advisory information
The sections that follow describe the message and TLV encodings for
these areas and the procedures that apply to them.
The label distribution procedures are complex and are difficult to
describe fully, coherently, and unambiguously as a collection of
separate message and TLV specifications.
Appendix A, "LDP Label Distribution Procedures", describes the label
distribution procedures in terms of label distribution events that
may occur at an LSR and how the LSR must respond. Appendix A is the
specification of LDP label distribution procedures. If a procedure
described elsewhere in this document conflicts with Appendix A,
Appendix A specifies LDP behavior.
3.3. Type-Length-Value Encoding
LDP uses a Type-Length-Value (TLV) encoding scheme to encode much of
the information carried in LDP messages.
An LDP TLV is encoded as a 2 octet field that uses 14 bits to specify
a Type and 2 bits to specify behavior when an LSR doesn't recognize
the Type, followed by a 2 octet Length field, followed by a variable
length Value field.
Andersson, et al. Standards Track [Page 31]

RFC 5036 LDP Specification October 2007
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Value |
~ ~
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
U-bit
Unknown TLV bit. Upon receipt of an unknown TLV, if U is clear
(=0), a notification MUST be returned to the message originator
and the entire message MUST be ignored; if U is set (=1), the
unknown TLV MUST be silently ignored and the rest of the message
processed as if the unknown TLV did not exist. The sections
following that define TLVs specify a value for the U-bit.
F-bit
Forward unknown TLV bit. This bit applies only when the U-bit is
set and the LDP message containing the unknown TLV is to be
forwarded. If F is clear (=0), the unknown TLV is not forwarded
with the containing message; if F is set (=1), the unknown TLV is
forwarded with the containing message. The sections following
that define TLVs specify a value for the F-bit. By setting both
the U- and F-bits, a TLV can be propagated as opaque data through
nodes that do not recognize the TLV.
Type
Encodes how the Value field is to be interpreted.
Length
Specifies the length of the Value field in octets.
Value
Octet string of Length octets that encodes information to be
interpreted as specified by the Type field.
Note that there is no alignment requirement for the first octet of a
TLV.
Note that the Value field itself may contain TLV encodings. That is,
TLVs may be nested.
Andersson, et al. Standards Track [Page 32]

RFC 5036 LDP Specification October 2007
The TLV encoding scheme is very general. In principle, everything
appearing in an LDP PDU could be encoded as a TLV. This
specification does not use the TLV scheme to its full generality. It
is not used where its generality is unnecessary and its use would
waste space unnecessarily. These are usually places where the type
of a value to be encoded is known, for example by its position in a
message or an enclosing TLV, and the length of the value is fixed or
readily derivable from the value encoding itself.
Some of the TLVs defined for LDP are similar to one another. For
example, there is a Generic Label TLV, an ATM Label TLV, and a Frame
Relay TLV; see Sections "Generic Label TLV", "ATM Label TLV", and
"Frame Relay TLV".
While it is possible to think about TLVs related in this way in terms
of a TLV type that specifies a TLV class and a TLV subtype that
specifies a particular kind of TLV within that class, this
specification does not formalize the notion of a TLV subtype.
The specification assigns type values for related TLVs, such as the
label TLVs, from a contiguous block in the 16-bit TLV type number
space.
Section "TLV Summary" lists the TLVs defined in this version of the
protocol and the section in this document that describes each.
3.4. TLV Encodings for Commonly Used Parameters
There are several parameters used by more than one LDP message. The
TLV encodings for these commonly used parameters are specified in
this section.
3.4.1. FEC TLV
Labels are bound to Forwarding Equivalence Classes (FECs). A FEC is
a list of one or more FEC elements. The FEC TLV encodes FEC items.
Andersson, et al. Standards Track [Page 33]

RFC 5036 LDP Specification October 2007
Its encoding is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| FEC (0x0100) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC Element 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC Element n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
FEC Element 1 to FEC Element n
There are several types of FEC elements; see Section "FECs". The
FEC element encoding depends on the type of FEC element.
A FEC Element value is encoded as a 1 octet field that specifies
the element type, and a variable length field that is the type-
dependent element value. Note that while the representation of
the FEC element value is type-dependent, the FEC element encoding
itself is one where standard LDP TLV encoding is not used.
The FEC Element value encoding is:
FEC Element Type Value
type name
Wildcard 0x01 No value; i.e., 0 value octets;
see below.
Prefix 0x02 See below.
Note that this version of LDP supports the use of multiple FEC
Elements per FEC for the Label Mapping message only. The use of
multiple FEC Elements in other messages is not permitted in this
version, and is a subject for future study.
Wildcard FEC Element
To be used only in the Label Withdraw and Label Release
messages. Indicates the withdraw/release is to be applied to
all FECs associated with the label within the following label
TLV. Must be the only FEC Element in the FEC TLV.
Andersson, et al. Standards Track [Page 34]

RFC 5036 LDP Specification October 2007
Prefix FEC Element value encoding:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix (2) | Address Family | PreLen |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Address Family
Two octet quantity containing a value from ADDRESS FAMILY
NUMBERS in [ASSIGNED_AF] that encodes the address family for
the address prefix in the Prefix field.
PreLen
One octet unsigned integer containing the length in bits of
the address prefix that follows. A length of zero indicates
a prefix that matches all addresses (the default
destination); in this case, the Prefix itself is zero
octets).
Prefix
An address prefix encoded according to the Address Family
field, whose length, in bits, was specified in the PreLen
field, padded to a byte boundary.
3.4.1.1. FEC Procedures
If in decoding a FEC TLV an LSR encounters a FEC Element with an
Address Family it does not support, it SHOULD stop decoding the FEC
TLV, abort processing the message containing the TLV, and send an
"Unsupported Address Family" Notification message to its LDP peer
signaling an error.
If it encounters a FEC Element type it cannot decode, it SHOULD stop
decoding the FEC TLV, abort processing the message containing the
TLV, and send an "Unknown FEC" Notification message to its LDP peer
signaling an error.
3.4.2. Label TLVs
Label TLVs encode labels. Label TLVs are carried by the messages
used to advertise, request, release, and withdraw label mappings.
There are several different kinds of Label TLVs that can appear in
situations that require a Label TLV.
Andersson, et al. Standards Track [Page 35]

RFC 5036 LDP Specification October 2007
VPI
Virtual Path Identifier. If VPI is less than 12-bits it SHOULD be
right justified in this field and preceding bits SHOULD be set to
0.
VCI
Virtual Channel Identifier. If the VCI is less than 16-bits, it
SHOULD be right justified in the field and the preceding bits MUST
be set to 0. If Virtual Path switching is indicated in the V-bits
field, then this field MUST be ignored by the receiver and set to
0 by the sender.
3.4.2.3. Frame Relay Label TLV
An LSR uses Frame Relay Label TLVs to encode labels for use on Frame
Relay links.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| Frame Relay Label (0x0202)| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Len| DLCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Res
This field is reserved. It MUST be set to zero on transmission
and MUST be ignored on receipt.
Len
This field specifies the number of bits of the DLCI. The
following values are supported:
0 = 10 bits of DLCI
2 = 23 bits of DLCI
Len values 1 and 3 are reserved.
DLCI
The Data Link Connection Identifier
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RFC 5036 LDP Specification October 2007
Addresses
A list of addresses from the specified Address Family. The
encoding of the individual addresses depends on the Address
Family.
The following address encodings are defined by this version of the
protocol:
Address Family Address Encoding
IPv4 4 octet full IPv4 address
IPv6 16 octet full IPv6 address
3.4.4. Hop Count TLV
The Hop Count TLV appears as an optional field in messages that set
up LSPs. It calculates the number of LSR hops along an LSP as the
LSP is being set up.
Note that setup procedures for LSPs that traverse ATM and Frame Relay
links require use of the Hop Count TLV (see [RFC3035] and [RFC3034]).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| Hop Count (0x0103) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HC Value |
+-+-+-+-+-+-+-+-+
HC Value
1 octet unsigned integer hop count value.
3.4.4.1. Hop Count Procedures
During setup of an LSP, an LSR R may receive a Label Mapping or Label
Request message for the LSP that contains the Hop Count TLV. If it
does, it SHOULD record the hop count value.
If LSR R then propagates the Label Mapping message for the LSP to an
upstream peer or the Label Request message to a downstream peer to
continue the LSP setup, it must determine a hop count to include in
the propagated message as follows:
- If the message is a Label Request message, R MUST increment the
received hop count;
Andersson, et al. Standards Track [Page 39]

RFC 5036 LDP Specification October 2007
- If the message is a Label Mapping message, R determines the hop
count as follows:
o If R is a member of the edge set of an LSR domain whose LSRs do
not perform 'TTL-decrement' and the upstream peer is within
that domain, R MUST reset the hop count to 1 before propagating
the message.
o Otherwise, R MUST increment the received hop count.
The first LSR in the LSP (ingress for a Label Request message, egress
for a Label Mapping message) SHOULD set the hop count value to 1.
By convention, a value of 0 indicates an unknown hop count. The
result of incrementing an unknown hop count is itself an unknown hop
count (0).
Use of the unknown hop count value greatly reduces the signaling
overhead when independent control is used. When a new LSP is
established, each LSR starts with an unknown hop count. Addition of
a new LSR whose hop count is also unknown does not cause a hop count
update to be propagated upstream since the hop count remains unknown.
When the egress is finally added to the LSP, then the LSRs propagate
hop count updates upstream via Label Mapping messages.
Without use of the unknown hop count, each time a new LSR is added to
the LSP a hop count update would need to be propagated upstream if
the new LSR is closer to the egress than any of the other LSRs.
These updates are useless overhead since they don't reflect the hop
count to the egress.
From the perspective of the ingress node, the fact that the hop count
is unknown implies nothing about whether a packet sent on the LSP
will actually make it to the egress. All it implies is that the hop
count update from the egress has not yet reached the ingress.
If an LSR receives a message containing a Hop Count TLV, it MUST
check the hop count value to determine whether the hop count has
exceeded its configured maximum allowable value. If so, it MUST
behave as if the containing message has traversed a loop by sending a
Notification message signaling Loop Detected in reply to the sender
of the message.
If Loop Detection is configured, the LSR MUST follow the procedures
specified in Section "Loop Detection".
Andersson, et al. Standards Track [Page 40]

RFC 5036 LDP Specification October 20073.4.5. Path Vector TLV
The Path Vector TLV is used with the Hop Count TLV in Label Request
and Label Mapping messages to implement the optional LDP Loop
Detection mechanism. See Section "Loop Detection". Its use in the
Label Request message records the path of LSRs the request has
traversed. Its use in the Label Mapping message records the path of
LSRs a label advertisement has traversed to set up an LSP. Its
encoding is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| Path Vector (0x0104) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LSR Id 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LSR Id n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
One or more LSR Ids
A list of router-ids indicating the path of LSRs the message has
traversed. Each LSR Id is the first four octets (router-id) of
the LDP Identifier for the corresponding LSR. This ensures it is
unique within the LSR network.
3.4.5.1. Path Vector Procedures
The Path Vector TLV is carried in Label Mapping and Label Request
messages when Loop Detection is configured.
3.4.5.1.1. Label Request Path Vector
Section "Loop Detection" specifies situations when an LSR must
include a Path Vector TLV in a Label Request message.
An LSR that receives a Path Vector in a Label Request message MUST
perform the procedures described in Section "Loop Detection".
If the LSR detects a loop, it MUST reject the Label Request message.
Andersson, et al. Standards Track [Page 41]

RFC 5036 LDP Specification October 2007
The LSR MUST:
1. Transmit a Notification message to the sending LSR signaling
"Loop Detected".
2. Not propagate the Label Request message further.
Note that a Label Request message with a Path Vector TLV is forwarded
until:
1. A loop is found,
2. The LSP egress is reached, or
3. The maximum Path Vector limit or maximum Hop Count limit is
reached. This is treated as if a loop had been detected.
3.4.5.1.2. Label Mapping Path Vector
Section "Loop Detection" specifies the situations when an LSR must
include a Path Vector TLV in a Label Mapping message.
An LSR that receives a Path Vector in a Label Mapping message MUST
perform the procedures described in Section "Loop Detection".
If the LSR detects a loop, it MUST reject the Label Mapping message
in order to prevent a forwarding loop. The LSR MUST:
1. Transmit a Label Release message carrying a Status TLV to the
sending LSR to signal "Loop Detected".
2. Not propagate the message further.
3. Check whether the Label Mapping message is for an existing LSP.
If so, the LSR must unsplice any upstream labels that are
spliced to the downstream label for the FEC.
Note that a Label Mapping message with a Path Vector TLV is forwarded
until:
1. A loop is found,
2. An LSP ingress is reached, or
3. The maximum Path Vector or maximum Hop Count limit is reached.
This is treated as if a loop had been detected.
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RFC 5036 LDP Specification October 2007
U-bit
Unknown message bit. Upon receipt of an unknown message, if U is
clear (=0), a notification is returned to the message originator;
if U is set (=1), the unknown message is silently ignored. The
sections following that define messages specify a value for the
U-bit.
Message Type
Identifies the type of message.
Message Length
Specifies the cumulative length in octets of the Message ID,
Mandatory Parameters, and Optional Parameters.
Message ID
32-bit value used to identify this message. Used by the sending
LSR to facilitate identifying Notification messages that may apply
to this message. An LSR sending a Notification message in
response to this message SHOULD include this Message ID in the
Status TLV carried by the Notification message; see Section
"Notification Message".
Mandatory Parameters
Variable length set of required message parameters. Some messages
have no required parameters.
For messages that have required parameters, the required
parameters MUST appear in the order specified by the individual
message specifications in the sections that follow.
Optional Parameters
Variable length set of optional message parameters. Many messages
have no optional parameters.
For messages that have optional parameters, the optional
parameters may appear in any order.
Note that there is no alignment requirement for the first octet of an
LDP message and that there is no padding at the end of a message;
that is, parameters can end at odd-byte boundaries.
Andersson, et al. Standards Track [Page 45]

RFC 5036 LDP Specification October 2007
The following message types are defined in this version of LDP:
Message Name Section Title
Notification "Notification Message"
Hello "Hello Message"
Initialization "Initialization Message"
KeepAlive "KeepAlive Message"
Address "Address Message"
Address Withdraw "Address Withdraw Message"
Label Mapping "Label Mapping Message"
Label Request "Label Request Message"
Label Abort Request "Label Abort Request Message"
Label Withdraw "Label Withdraw Message"
Label Release "Label Release Message"
The sections that follow specify the encodings and procedures for
these messages.
Some of the above messages are related to one another, for example
the Label Mapping, Label Request, Label Withdraw, and Label Release
messages.
While it is possible to think about messages related in this way in
terms of a message type that specifies a message class and a message
subtype that specifies a particular kind of message within that
class, this specification does not formalize the notion of a message
subtype.
The specification assigns type values for related messages, such as
the Label messages, from of a contiguous block in the 16-bit message
type number space.
3.5.1. Notification Message
An LSR sends a Notification message to inform an LDP peer of a
significant event. A Notification message signals a fatal error or
provides advisory information such as the outcome of processing an
LDP message or the state of the LDP session.
Andersson, et al. Standards Track [Page 46]

RFC 5036 LDP Specification October 2007
The encoding for the Notification message is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Notification (0x0001) | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status (TLV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Parameters |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message ID
32-bit value used to identify this message.
Status TLV
Indicates the event being signaled. The encoding for the Status
TLV is specified in Section "Status TLV".
Optional Parameters
This variable length field contains 0 or more parameters, each
encoded as a TLV. The following Optional Parameters are generic
and may appear in any Notification message:
Optional Parameter Type Length Value
Extended Status 0x0301 4 See below
Returned PDU 0x0302 var See below
Returned Message 0x0303 var See below
Other Optional Parameters, specific to the particular event being
signaled by the Notification messages, may appear. These are
described elsewhere.
Extended Status
The 4 octet value is an Extended Status Code that encodes
additional information that supplements the status information
contained in the Notification Status Code.
Returned PDU
An LSR uses this parameter to return part of an LDP PDU to the LSR
that sent it. The value of this TLV is the PDU header and as much
PDU data following the header as appropriate for the condition
being signaled by the Notification message.
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RFC 5036 LDP Specification October 2007
Returned Message
An LSR uses this parameter to return part of an LDP message to the
LSR that sent it. The value of this TLV is the message type and
length fields and as much message data following the type and
length fields as appropriate for the condition being signaled by
the Notification message.
3.5.1.1. Notification Message Procedures
If an LSR encounters a condition requiring it to notify its peer with
advisory or error information, it sends the peer a Notification
message containing a Status TLV that encodes the information and
optionally additional TLVs that provide more information about the
condition.
If the condition is one that is a fatal error, the Status Code
carried in the Notification will indicate that. In this case, after
sending the Notification message the LSR SHOULD terminate the LDP
session by closing the session TCP connection and discard all state
associated with the session, including all label-FEC bindings learned
via the session.
When an LSR receives a Notification message that carries a Status
Code that indicates a fatal error, it SHOULD terminate the LDP
session immediately by closing the session TCP connection and discard
all state associated with the session, including all label-FEC
bindings learned via the session.
The above statement does not apply to the processing of the Shutdown
message in the session initialization procedure. When an LSR
receives a Shutdown message during session initialization, it SHOULD
transmit a Shutdown message and then close the transport connection.
3.5.1.2. Events Signaled by Notification Messages
It is useful for descriptive purpose to classify events signaled by
Notification messages into the following categories.
3.5.1.2.1. Malformed PDU or Message
Malformed LDP PDUs or messages that are part of the LDP Discovery
mechanism are handled by silently discarding them.
An LDP PDU received on a TCP connection for an LDP session is
malformed if:
Andersson, et al. Standards Track [Page 48]

RFC 5036 LDP Specification October 2007
- The LDP Identifier in the PDU header is unknown to the
receiver, or it is known but is not the LDP Identifier
associated by the receiver with the LDP peer for this LDP
session. This is a fatal error signaled by the Bad LDP
Identifier Status Code.
- The LDP protocol version is not supported by the receiver, d or
it is supported but is not the version negotiated for the
session during session establishment. This is a fatal error
signaled by the Bad Protocol Version Status Code.
- The PDU Length field is too small (< 14) or too large (>
maximum PDU length). This is a fatal error signaled by the Bad
PDU Length Status Code. Section "Initialization Message"
describes how the maximum PDU length for a session is
determined.
An LDP message is malformed if:
- The Message Type is unknown.
If the Message Type is < 0x8000 (high order bit = 0), it is an
error signaled by the Unknown Message Type Status Code.
If the Message Type is >= 0x8000 (high order bit = 1), it is
silently discarded.
- The Message Length is too large, that is, indicates that the
message extends beyond the end of the containing LDP PDU. This
is a fatal error signaled by the Bad Message Length Status
Code.
- The Message Length is too small, that is, smaller than the
smallest possible value component. This is a fatal error
signaled by the Bad Message Length Status Code.
- The message is missing one or more Mandatory Parameters. This
is a non-fatal error signaled by the Missing Message Parameters
Status Code.
3.5.1.2.2. Unknown or Malformed TLV
Malformed TLVs contained in LDP messages that are part of the LDP
Discovery mechanism are handled by silently discarding the containing
message.
A TLV contained in an LDP message received on a TCP connection of an
LDP is malformed if:
Andersson, et al. Standards Track [Page 49]

RFC 5036 LDP Specification October 2007
- The TLV Length is too large, that is, indicates that the TLV
extends beyond the end of the containing message. This is a
fatal error signaled by the Bad TLV Length Status Code.
- The TLV type is unknown.
If the TLV type is < 0x8000 (high order bit = 0), it is an
error signaled by the Unknown TLV Status Code.
If the TLV type is >= 0x8000 (high order bit = 1), the TLV is
silently dropped.
- The TLV Value is malformed. This occurs when the receiver
handles the TLV but cannot decode the TLV Value. This is
interpreted as indicative of a bug in either the sending or
receiving LSR. It is a fatal error signaled by the Malformed
TLV Value Status Code.
3.5.1.2.3. Session KeepAlive Timer Expiration
This is a fatal error signaled by the KeepAlive Timer Expired Status
Code.
3.5.1.2.4. Unilateral Session Shutdown
This is a fatal event signaled by the Shutdown Status Code. The
Notification message may optionally include an Extended Status TLV to
provide a reason for the Shutdown. The sending LSR terminates the
session immediately after sending the Notification.
3.5.1.2.5. Initialization Message Events
The session initialization negotiation (see Section "Session
Initialization") may fail if the session parameters received in the
Initialization message are unacceptable. This is a fatal error. The
specific Status Code depends on the parameter deemed unacceptable,
and is defined in Sections "Initialization Message".
3.5.1.2.6. Events Resulting from Other Messages
Messages other than the Initialization message may result in events
that must be signaled to LDP peers via Notification messages. These
events and the Status Codes used in the Notification messages to
signal them are described in the sections that describe these
messages.
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RFC 5036 LDP Specification October 2007
Hold Time
Hello hold time in seconds. An LSR maintains a record of
Hellos received from potential peers (see Section "Hello
Message Procedures"). Hello Hold Time specifies the time the
sending LSR will maintain its record of Hellos from the
receiving LSR without receipt of another Hello.
A pair of LSRs negotiates the hold times they use for Hellos
from each other. Each proposes a hold time. The hold time
used is the minimum of the hold times proposed in their Hellos.
A value of 0 means use the default, which is 15 seconds for
Link Hellos and 45 seconds for Targeted Hellos. A value of
0xffff means infinite.
T, Targeted Hello
A value of 1 specifies that this Hello is a Targeted Hello. A
value of 0 specifies that this Hello is a Link Hello.
R, Request Send Targeted Hellos
A value of 1 requests the receiver to send periodic Targeted
Hellos to the source of this Hello. A value of 0 makes no
request.
An LSR initiating Extended Discovery sets R to 1. If R is 1,
the receiving LSR checks whether it has been configured to send
Targeted Hellos to the Hello source in response to Hellos with
this request. If not, it ignores the request. If so, it
initiates periodic transmission of Targeted Hellos to the Hello
source.
Reserved
This field is reserved. It MUST be set to zero on transmission
and ignored on receipt.
Optional Parameters
This variable length field of the Hello message contains 0 or more
parameters, each encoded as a TLV. The optional parameters
defined by this version of the protocol are
Optional Parameter Type Length Value
IPv4 Transport Address 0x0401 4 See below
Configuration 0x0402 4 See below
Sequence Number
IPv6 Transport Address 0x0403 16 See below
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RFC 5036 LDP Specification October 2007
IPv4 Transport Address
Specifies the IPv4 address to be used for the sending LSR when
opening the LDP session TCP connection. If this optional TLV
is not present, the IPv4 source address for the UDP packet
carrying the Hello SHOULD be used.
Configuration Sequence Number
Specifies a 4 octet unsigned configuration sequence number that
identifies the configuration state of the sending LSR. Used by
the receiving LSR to detect configuration changes on the
sending LSR.
IPv6 Transport Address
Specifies the IPv6 address to be used for the sending LSR when
opening the LDP session TCP connection. If this optional TLV
is not present the IPv6 source address for the UDP packet
carrying the Hello SHOULD be used.
3.5.2.1. Hello Message Procedures
An LSR receiving Hellos from another LSR maintains a Hello adjacency
corresponding to the Hellos. The LSR maintains a hold timer with the
Hello adjacency, which it restarts whenever it receives a Hello that
matches the Hello adjacency. If the hold timer for a Hello adjacency
expires the LSR discards the Hello adjacency: see Sections
"Maintaining Hello Adjacencies" and "Maintaining LDP Sessions".
We recommend that the interval between Hello transmissions be at most
one third of the Hello hold time.
An LSR processes a received LDP Hello as follows:
1. The LSR checks whether the Hello is acceptable. The criteria
for determining whether a Hello is acceptable are
implementation dependent (see below for example criteria).
2. If the Hello is not acceptable, the LSR ignores it.
3. If the Hello is acceptable, the LSR checks whether it has a
Hello adjacency for the Hello source. If so, it restarts the
hold timer for the Hello adjacency. If not, it creates a Hello
adjacency for the Hello source and starts its hold timer.
4. If the Hello carries any optional TLVs, the LSR processes them
(see below).
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RFC 5036 LDP Specification October 2007
5. Finally, if the LSR has no LDP session for the label space
specified by the LDP Identifier in the PDU header for the
Hello, it follows the procedures of Section "LDP Session
Establishment".
The following are examples of acceptability criteria for Link and
Targeted Hellos:
A Link Hello is acceptable if the interface on which it was
received has been configured for label switching.
A Targeted Hello from source address A is acceptable if either:
- The LSR has been configured to accept Targeted Hellos, or
- The LSR has been configured to send Targeted Hellos to A.
The following describes how an LSR processes Hello optional TLVs:
Transport Address
The LSR associates the specified transport address with the
Hello adjacency.
Configuration Sequence Number
The Configuration Sequence Number optional parameter is used by
the sending LSR to signal configuration changes to the
receiving LSR. When a receiving LSR playing the active role in
LDP session establishment detects a change in the sending LSR
configuration, it may clear the session setup backoff delay, if
any, associated with the sending LSR (see Section "Session
Initialization").
A sending LSR using this optional parameter is responsible for
maintaining the configuration sequence number it transmits in
Hello messages. Whenever there is a configuration change on
the sending LSR, it increments the configuration sequence
number.
3.5.3. Initialization Message
The LDP Initialization message is exchanged as part of the LDP
session establishment procedure; see Section "LDP Session
Establishment".
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RFC 5036 LDP Specification October 2007
value chosen for KeepAlive Time indicates the maximum number of
seconds that may elapse between the receipt of successive PDUs
from the LDP peer on the session TCP connection. The KeepAlive
Timer is reset each time a PDU arrives.
A, Label Advertisement Discipline
Indicates the type of Label advertisement. A value of 0 means
Downstream Unsolicited advertisement; a value of 1 means
Downstream On Demand.
If one LSR proposes Downstream Unsolicited and the other
proposes Downstream on Demand, the rules for resolving this
difference is:
- If the session is for a label-controlled ATM link or a
label-controlled Frame Relay link, then Downstream on Demand
MUST be used.
- Otherwise, Downstream Unsolicited MUST be used.
If the label advertisement discipline determined in this way
is unacceptable to an LSR, it MUST send a Session
Rejected/Parameters Advertisement Mode Notification message
in response to the Initialization message and not establish
the session.
D, Loop Detection
Indicates whether Loop Detection based on Path Vectors is
enabled. A value of 0 means that Loop Detection is disabled; a
value of 1 means that Loop Detection is enabled.
PVLim, Path Vector Limit
The configured maximum Path Vector length. MUST be 0 if Loop
Detection is disabled (D = 0). If the Loop Detection
procedures would require the LSR to send a Path Vector that
exceeds this limit, the LSR will behave as if a loop had been
detected for the FEC in question.
When Loop Detection is enabled in a portion of a network, it is
recommended that all LSRs in that portion of the network be
configured with the same Path Vector limit. Although knowledge
of a peer's Path Vector limit will not change an LSR's
behavior, it does enable the LSR to alert an operator to a
possible misconfiguration.
Reserved
This field is reserved. It MUST be set to zero on transmission
and ignored on receipt.
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RFC 5036 LDP Specification October 2007
Max PDU Length
Two octet unsigned integer that proposes the maximum allowable
length for LDP PDUs for the session. A value of 255 or less
specifies the default maximum length of 4096 octets.
The receiving LSR MUST calculate the maximum PDU length for the
session by using the smaller of its and its peer's proposals
for Max PDU Length. The default maximum PDU length applies
before session initialization completes.
If the maximum PDU length determined this way is unacceptable
to an LSR, it MUST send a Session Rejected/Parameters Max PDU
Length Notification message in response to the Initialization
message and not establish the session.
Receiver LDP Identifier
Identifies the receiver's label space. This LDP Identifier,
together with the sender's LDP Identifier in the PDU header,
enables the receiver to match the Initialization message with
one of its Hello adjacencies; see Section "Hello Message
Procedures".
If there is no matching Hello adjacency, the LSR MUST send a
Session Rejected/No Hello Notification message in response to
the Initialization message and not establish the session.
Optional Parameters
This variable length field contains 0 or more parameters, each
encoded as a TLV. The optional parameters are:
Optional Parameter Type Length Value
ATM Session Parameters 0x0501 var See below
Frame Relay Session 0x0502 var See below
Parameters
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RFC 5036 LDP Specification October 2007
D, VC Directionality
A value of 0 specifies bidirectional VC capability, meaning the
LSR can (within a given VPI) support the use of a given VCI as
a label for both link directions independently. A value of 1
specifies unidirectional VC capability, meaning (within a given
VPI) a given VCI may appear in a label mapping for one
direction on the link only. When either or both of the peers
specifies unidirectional VC capability, both LSRs use
unidirectional VC label assignment for the link as follows.
The LSRs compare their LDP Identifiers as unsigned integers.
The LSR with the larger LDP Identifier may assign only odd-
numbered VCIs in the VPI/VCI range as labels. The system with
the smaller LDP Identifier may assign only even-numbered VCIs
in the VPI/VCI range as labels.
Reserved
This field is reserved. It MUST be set to zero on transmission
and ignored on receipt.
One or more ATM Label Range Components
A list of ATM Label Range Components that together specify the
Label range supported by the transmitting LSR.
A receiving LSR MUST calculate the intersection between the
received range and its own supported label range. The
intersection is the range in which the LSR may allocate and
accept labels. LSRs MUST NOT establish a session with
neighbors for which the intersection of ranges is NULL. In
this case, the LSR MUST send a Session Rejected/Parameters
Label Range Notification message in response to the
Initialization message and not establish the session.
The encoding for an ATM Label Range Component is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Res | Minimum VPI | Minimum VCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Res | Maximum VPI | Maximum VCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Res
This field is reserved. It MUST be set to zero on
transmission and MUST be ignored on receipt.
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RFC 5036 LDP Specification October 2007
Minimum VPI (12 bits)
This 12-bit field specifies the lower bound of a block of
Virtual Path Identifiers that is supported on the
originating switch. If the VPI is less than 12 bits, it
SHOULD be right justified in this field and preceding bits
SHOULD be set to 0.
Minimum VCI (16 bits)
This 16-bit field specifies the lower bound of a block of
Virtual Channel Identifiers that is supported on the
originating switch. If the VCI is less than 16 bits, it
SHOULD be right justified in this field and preceding bits
SHOULD be set to 0.
Maximum VPI (12 bits)
This 12-bit field specifies the upper bound of a block of
Virtual Path Identifiers that is supported on the
originating switch. If the VPI is less than 12 bits, it
SHOULD be right justified in this field and preceding bits
SHOULD be set to 0.
Maximum VCI (16 bits)
This 16-bit field specifies the upper bound of a block of
Virtual Connection Identifiers that is supported on the
originating switch. If the VCI is less than 16 bits, it
SHOULD be right justified in this field and preceding bits
SHOULD be set to 0.
When peer LSRs are connected indirectly by means of an ATM VP, the
sending LSR SHOULD set the Minimum and Maximum VPI fields to 0,
and the receiving LSR MUST ignore the Minimum and Maximum VPI
fields.
Frame Relay Session Parameters
Used when an LDP session manages label exchange for a Frame
Relay link to specify Frame Relay-specific session parameters.
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RFC 5036 LDP Specification October 2007
Reserved
This field is reserved. It MUST be set to zero on transmission
and ignored on receipt.
One or more Frame Relay Label Range Components
A list of Frame Relay Label Range Components that together
specify the Label range supported by the transmitting LSR.
A receiving LSR MUST calculate the intersection between the
received range and its own supported label range. The
intersection is the range in which the LSR may allocate and
accept labels. LSRs MUST NOT establish a session with
neighbors for which the intersection of ranges is NULL. In
this case, the LSR MUST send a Session Rejected/Parameters
Label Range Notification message in response to the
Initialization message and not establish the session.
The encoding for a Frame Relay Label Range Component is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Len| Minimum DLCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Maximum DLCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved
This field is reserved. It MUST be set to zero on transmission
and ignored on receipt.
Len
This field specifies the number of bits of the DLCI. The
following values are supported:
Len DLCI Bits
0 10
2 23
Len values 1 and 3 are reserved.
Minimum DLCI
This 23-bit field specifies the lower bound of a block of Data
Link Connection Identifiers (DLCIs) that is supported on the
originating switch. The DLCI SHOULD be right justified in this
field and unused bits SHOULD be set to 0.
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Maximum DLCI
This 23-bit field specifies the upper bound of a block of Data
Link Connection Identifiers (DLCIs) that is supported on the
originating switch. The DLCI SHOULD be right justified in this
field and unused bits SHOULD be set to 0.
Note that there is no Generic Session Parameters TLV for sessions
that advertise Generic Labels.
3.5.3.1. Initialization Message Procedures
See Section "LDP Session Establishment" and particularly Section
"Session Initialization" for general procedures for handling the
Initialization message.
3.5.4. KeepAlive Message
An LSR sends KeepAlive messages as part of a mechanism that monitors
the integrity of the LDP session transport connection.
The encoding for the KeepAlive message is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| KeepAlive (0x0201) | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Parameters |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message ID
32-bit value used to identify this message.
Optional Parameters
No optional parameters are defined for the KeepAlive message.
3.5.4.1. KeepAlive Message Procedures
The KeepAlive Timer mechanism described in Section "Maintaining LDP
Sessions" resets a session KeepAlive Timer every time an LDP PDU is
received on the session TCP connection. The KeepAlive message is
provided to allow reset of the KeepAlive Timer in circumstances where
an LSR has no other information to communicate to an LDP peer.
An LSR MUST arrange that its peer receive an LDP message from it at
least every KeepAlive Time period. Any LDP protocol message will do
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Optional Parameter Length Value
Label Request 4 See below
Message ID TLV
Hop Count TLV 1 See below
Path Vector TLV variable See below
The encodings for the Hop Count and Path Vector TLVs can be found
in Section "TLV Encodings for Commonly Used Parameters".
Label Request Message ID
If this Label Mapping message is a response to a Label Request
message, it MUST include the Label Request Message ID optional
parameter. The value of this optional parameter is the Message
ID of the corresponding Label Request message.
Hop Count
Specifies the running total of the number of LSR hops along the
LSP being set up by the Label message. Section "Hop Count
Procedures" describes how to handle this TLV.
Path Vector
Specifies the LSRs along the LSP being set up by the Label
message. Section "Path Vector Procedures" describes how to
handle this TLV.
3.5.7.1. Label Mapping Message Procedures
The Mapping message is used by an LSR to distribute a label mapping
for a FEC to an LDP peer. If an LSR distributes a mapping for a FEC
to multiple LDP peers, it is a local matter whether it maps a single
label to the FEC, and distributes that mapping to all its peers, or
whether it uses a different mapping for each of its peers.
An LSR is responsible for the consistency of the label mappings it
has distributed and that its peers have these mappings.
An LSR receiving a Label Mapping message from a downstream LSR for a
Prefix SHOULD NOT use the label for forwarding unless its routing
table contains an entry that exactly matches the FEC Element.
See Appendix A, "LDP Label Distribution Procedures", for more
details.
3.5.7.1.1. Independent Control Mapping
If an LSR is configured for independent control, a mapping message is
transmitted by the LSR upon any of the following conditions:
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RFC 5036 LDP Specification October 2007
1. The LSR recognizes a new FEC via the forwarding table, and the
label advertisement mode is Downstream Unsolicited
advertisement.
2. The LSR receives a Request message from an upstream peer for a
FEC present in the LSR's forwarding table.
3. The next hop for a FEC changes to another LDP peer, and Loop
detection is configured.
4. The attributes of a mapping change.
5. The receipt of a mapping from the downstream next hop AND
a) no upstream mapping has been created OR
b) loop detection is configured OR
c) the attributes of the mapping have changed.
3.5.7.1.2. Ordered Control Mapping
If an LSR is doing Ordered Control, a Mapping message is transmitted
by downstream LSRs upon any of the following conditions:
1. The LSR recognizes a new FEC via the forwarding table and is
the egress for that FEC.
2. The LSR receives a Request message from an upstream peer for a
FEC present in the LSR's forwarding table, and the LSR is the
egress for that FEC OR has a downstream mapping for that FEC.
3. The next hop for a FEC changes to another LDP peer, and Loop
Detection is configured.
4. The attributes of a mapping change.
5. The receipt of a mapping from the downstream next hop AND
a) no upstream mapping has been created OR
b) Loop Detection is configured OR
c) the attributes of the mapping have changed.
3.5.7.1.3. Downstream on Demand Label Advertisement
In general, the upstream LSR is responsible for requesting label
mappings when operating in Downstream on Demand mode. However,
unless some rules are followed, it is possible for neighboring LSRs
with different advertisement modes to get into a livelock situation
where everything is functioning properly, but no labels are
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RFC 5036 LDP Specification October 2007
distributed. For example, consider two LSRs Ru and Rd where Ru is
the upstream LSR and Rd is the downstream LSR for a particular FEC.
In this example, Ru is using Downstream Unsolicited advertisement
mode and Rd is using Downstream on Demand mode. In this case, Rd may
assume that Ru will request a label mapping when it wants one and Ru
may assume that Rd will advertise a label if it wants Ru to use one.
If Rd and Ru operate as suggested, no labels will be distributed from
Rd to Ru.
This livelock situation can be avoided if the following rule is
observed: an LSR operating in Downstream on Demand mode SHOULD NOT be
expected to send unsolicited mapping advertisements. Therefore, if
the downstream LSR is operating in Downstream on Demand mode, the
upstream LSR is responsible for requesting label mappings as needed.
3.5.7.1.4. Downstream Unsolicited Label Advertisement
In general, the downstream LSR is responsible for advertising a label
mapping when it wants an upstream LSR to use the label. An upstream
LSR may issue a mapping request if it so desires.
The combination of Downstream Unsolicited mode and Conservative Label
retention can lead to a situation where an LSR releases the label for
a FEC that it later needs. For example, if LSR Rd advertises to LSR
Ru the label for a FEC for which it is not Ru's next hop, Ru will
release the label. If Ru's next hop for the FEC later changes to Rd,
it needs the previously released label.
To deal with this situation, either Ru can explicitly request the
label when it needs it, or Rd can periodically re-advertise it to Ru.
In many situations Ru will know when it needs the label from Rd. For
example, when its next hop for the FEC changes to Rd. However, there
could be situations when Ru does not. For example, Rd may be
attempting to establish an LSP with non-standard properties. Forcing
Ru to explicitly request the label in this situation would require it
to maintain state about a potential LSP with non-standard properties.
In situations where Ru knows it needs the label, it is responsible
for explicitly requesting the label by means of a Label Request
message. In situations where Ru may not know that it needs the
label, Rd is responsible for periodically re-advertising the label to
Ru.
For this version of LDP, the only situation where Ru knows it needs a
label for a FEC from Rd is when Rd is its next hop for the FEC, Ru
does not have a label from Rd, and the LSP for the FEC is one that
can be established with TLVs defined in this document.
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RFC 5036 LDP Specification October 20073.5.8.1. Label Request Message Procedures
The Request message is used by an upstream LSR to explicitly request
that the downstream LSR assign and advertise a label for a FEC.
An LSR may transmit a Request message under any of the following
conditions:
1. The LSR recognizes a new FEC via the forwarding table, and the
next hop is an LDP peer, and the LSR doesn't already have a
mapping from the next hop for the given FEC.
2. The next hop to the FEC changes, and the LSR doesn't already
have a mapping from that next hop for the given FEC.
Note that if the LSR already has a pending Label Request
message for the new next hop, it SHOULD NOT issue an additional
Label Request in response to the next hop change.
3. The LSR receives a Label Request for a FEC from an upstream LDP
peer, the FEC next hop is an LDP peer, and the LSR doesn't
already have a mapping from the next hop.
Note that since a non-merge LSR must set up a separate LSP for
each upstream peer requesting a label, it must send a separate
Label Request for each such peer. A consequence of this is
that a non-merge LSR may have multiple Label Request messages
for a given FEC outstanding at the same time.
The receiving LSR SHOULD respond to a Label Request message with a
Label Mapping for the requested label or with a Notification message
indicating why it cannot satisfy the request.
When the FEC for which a label is requested is a Prefix FEC Element,
the receiving LSR uses its routing table to determine its response.
Unless its routing table includes an entry that exactly matches the
requested Prefix, the LSR MUST respond with a No Route Notification
message.
The message ID of the Label Request message serves as an identifier
for the Label Request transaction. When the receiving LSR responds
with a Label Mapping message, the mapping message MUST include a
Label Request/Returned Message ID TLV optional parameter that
includes the message ID of the Label Request message. Note that
since LSRs use Label Request message IDs as transaction identifiers,
an LSR SHOULD NOT reuse the message ID of a Label Request message
until the corresponding transaction completes.
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FEC TLV
Identifies the FEC for which the Label Request is being aborted.
Label Request Message ID TLV
Specifies the message ID of the Label Request message to be
aborted.
Optional Parameters
No optional parameters are defined for the Label Abort Req
message.
3.5.9.1. Label Abort Request Message Procedures
An LSR Ru may send a Label Abort Request message to abort an
outstanding Label Request message for a FEC sent to an LSR Rd in the
following circumstances:
1. Ru's next hop for the FEC has changed from LSR Rd to LSR X; or
2. Ru is a non-merge, non-ingress LSR and has received a Label
Abort Request for the FEC from an upstream peer Y.
3. Ru is a merge, non-ingress LSR and has received a Label Abort
Request for the FEC from an upstream peer Y and Y is the only
(last) upstream LSR requesting a label for the FEC.
There may be other situations where an LSR may choose to abort an
outstanding Label Request message in order to reclaim resource
associated with the pending LSP. However, specification of general
strategies for using the abort mechanism is beyond the scope of LDP.
When an LSR receives a Label Abort Request message, if it has not
previously responded to the Label Request being aborted with a Label
Mapping message or some other Notification message, it MUST
acknowledge the abort by responding with a Label Request Aborted
Notification message. The Notification MUST include a Label Request
Message ID TLV that carries the message ID of the aborted Label
Request message.
If an LSR receives a Label Abort Request Message after it has
responded to the Label Request in question with a Label Mapping
message or a Notification message, it ignores the abort request.
If an LSR receives a Label Mapping message in response to a Label
Request message after it has sent a Label Abort Request message to
abort the Label Request, the label in the Label Mapping message is
valid. The LSR may choose to use the label or to release it with a
Label Release message.
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An LSR aborting a Label Request message may not reuse the Message ID
for the Label Request message until it receives one of the following
from its peer:
- A Label Request Aborted Notification message acknowledging the
abort;
- A Label Mapping message in response to the Label Request
message being aborted;
- A Notification message in response to the Label Request message
being aborted (e.g., Loop Detected, No Label Resources, etc.).
To protect itself against tardy peers or faulty peer implementations
an LSR may choose to time out receipt of the above. The timeout
period should be relatively long (several minutes). If the timeout
period elapses with no reply from the peer, the LSR may reuse the
Message ID of the Label Request message; if it does so, it should
also discard any record of the outstanding Label Request and Label
Abort messages.
Note that the response to a Label Abort Request message is never
"ordered". That is, the response does not depend on the downstream
state of the LSP setup being aborted. An LSR receiving a Label Abort
Request message MUST process it immediately, regardless of the
downstream state of the LSP, responding with a Label Request Aborted
Notification or ignoring it, as appropriate.
3.5.10. Label Withdraw Message
An LSR sends a Label Withdraw Message to an LDP peer to signal the
peer that the peer may not continue to use specific FEC-label
mappings the LSR had previously advertised. This breaks the mapping
between the FECs and the labels.
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RFC 5036 LDP Specification October 2007
The FEC TLV specifies the FEC for which labels are to be withdrawn.
If no Label TLV follows the FEC, all labels associated with the FEC
are to be withdrawn; otherwise, only the label specified in the
optional Label TLV is to be withdrawn.
The FEC TLV may contain the Wildcard FEC Element; if so, it may
contain no other FEC Elements. In this case, if the Label Withdraw
message contains an optional Label TLV, then the label is to be
withdrawn from all FECs to which it is bound. If there is not an
optional Label TLV in the Label Withdraw message, then the sending
LSR is withdrawing all label mappings previously advertised to the
receiving LSR.
An LSR that receives a Label Withdraw message MUST respond with a
Label Release message.
See Appendix A, "LDP Label Distribution Procedures", for more
details.
3.5.11. Label Release Message
An LSR sends a Label Release message to an LDP peer to signal the
peer that the LSR no longer needs specific FEC-label mappings
previously requested of and/or advertised by the peer.
The encoding for the Label Release Message is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Label Release (0x0403) | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label TLV (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Parameters |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message ID
32-bit value used to identify this message.
FEC TLV
Identifies the FEC for which the FEC-label mapping is being
released.
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RFC 5036 LDP Specification October 2007
Optional Parameters
This variable length field contains 0 or more parameters, each
encoded as a TLV. The optional parameters are:
Optional Parameter Length Value
Label TLV variable See below
The encodings for Label TLVs are found in Section "Label TLVs".
Label
If present, the label being released (see procedures below).
3.5.11.1. Label Release Message Procedures
An LSR transmits a Label Release message to a peer when it no longer
needs a label previously received from or requested of that peer.
An LSR MUST transmit a Label Release message under any of the
following conditions:
1. The LSR that sent the label mapping is no longer the next hop
for the mapped FEC, and the LSR is configured for conservative
operation.
2. The LSR receives a label mapping from an LSR that is not the
next hop for the FEC, and the LSR is configured for
conservative operation.
3. The LSR receives a Label Withdraw message.
Note that if an LSR is configured for "liberal mode", a release
message will never be transmitted in the case of conditions (1) and
(2) as specified above. In this case, the upstream LSR keeps each
unused label, so that it can immediately be used later if the
downstream peer becomes the next hop for the FEC.
The FEC TLV specifies the FEC for which labels are to be released.
If no Label TLV follows the FEC, all labels associated with the FEC
are to be released; otherwise, only the label specified in the
optional Label TLV is to be released.
The FEC TLV may contain the Wildcard FEC Element; if so, it may
contain no other FEC Elements. In this case, if the Label Release
message contains an optional Label TLV, then the label is to be
released for all FECs to which it is bound. If there is not an
optional Label TLV in the Label Release message, then the sending LSR
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RFC 5036 LDP Specification October 2007
The determination as to whether a vendor-private message is
understood is based on the Type and the mandatory Vendor ID field.
Implementations that support vendor-private TLVs MUST support a
user-accessible configuration interface that causes the U-bit to
be set on all transmitted vendor-private TLVs; this requirement
MAY be satisfied by a user-accessible configuration interface that
prevents transmission of all vendor-private TLVs for which the U-
bit is clear.
F-bit
Forward unknown TLV bit. This bit only applies when the U-bit is
set and the LDP message containing the unknown TLV is to be
forwarded. If F is clear (=0), the unknown TLV is not forwarded
with the containing message; if F is set (=1), the unknown TLV is
forwarded with the containing message.
Type
Type value in the range 0x3E00 through 0x3EFF. Together, the Type
and Vendor ID field specify how the Data field is to be
interpreted.
Length
Specifies the cumulative length in octets of the Vendor ID and
Data fields.
Vendor ID
802 Vendor ID as assigned by the IEEE.
Data
The remaining octets after the Vendor ID in the Value field are
optional vendor-dependent data.
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RFC 5036 LDP Specification October 2007
Message Length
Specifies the cumulative length in octets of the Message ID,
Vendor ID, Remaining Mandatory Parameters, and Optional
Parameters.
Message ID
32-bit integer used to identify this message. Used by the sending
LSR to facilitate identifying Notification messages that may apply
to this message. An LSR sending a Notification message in
response to this message will include this Message ID in the
notification message; see Section "Notification Message".
Vendor ID
802 Vendor ID as assigned by the IEEE.
Remaining Mandatory Parameters
Variable length set of remaining required message parameters.
Optional Parameters
Variable length set of optional message parameters.
3.6.2. LDP Experimental Extensions
LDP support for experimentation is similar to support for vendor-
private extensions with the following differences:
- The Type range 0x3F00 through 0x3FFF is reserved for
experimental TLVs.
- The Message Type range 0x3F00 through 0x3FFF is reserved for
experimental messages.
- The encodings for experimental TLVs and messages are similar to
the vendor-private encodings with the following difference.
Experimental TLVs and messages use an Experiment ID field in
place of a Vendor ID field. The Experiment ID field is used
with the Type or Message Type field to specify the
interpretation of the experimental TLV or Message.
Administration of Experiment IDs is the responsibility of the
experimenters.
3.7. Message Summary
The following are the LDP messages defined in this version of the
protocol.
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Unsupported Address 0 0x00000017 "FEC Procedures"
Family "Address Message Proc ..."
Session Rejected/ 1 0x00000018 "Session Initialization"
Bad KeepAlive Time
Internal Error 1 0x00000019 "Events Signaled by ..."
3.10. Well-Known Numbers3.10.1. UDP and TCP Ports
The UDP port for LDP Hello messages is 646.
The TCP port for establishing LDP session connections is 646.
3.10.2. Implicit NULL Label
The Implicit NULL label is defined in [RFC3031] as follows:
"The Implicit NULL label is a label with special semantics which an
LSR can bind to an address prefix. If LSR Ru, by consulting its ILM
(Incoming Label Map) sees that labeled packet P must be forwarded
next to Rd, but that Rd has distributed a binding of Implicit NULL to
the corresponding address prefix, then instead of replacing the value
of the label on top of the label stack, Ru pops the label stack, and
then forwards the resulting packet to Rd."
The implicit NULL label is represented in LDP as a Generic Label TLV
with a Label field value of 3, as defined in [RFC3032].
4. IANA Considerations
LDP defines the following name spaces that require management:
- Message Type Name Space
- TLV Type Name Space
- FEC Type Name Space
- Status Code Name Space
- Experiment ID Name Space
The following sections provide guidelines for managing these name
spaces.
4.1. Message Type Name Space
LDP divides the name space for message types into three ranges. The
following are the guidelines for managing these ranges:
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- Message Types 0x0000 - 0x3DFF. Message types in this range are
part of the LDP base protocol. Following the policies outlined
in [IANA], Message types in this range are allocated through an
IETF Consensus action.
- Message Types 0x3E00 - 0x3EFF. Message types in this range are
reserved for Vendor-Private extensions and are the
responsibility of the individual vendors (see Section "LDP
Vendor-Private Messages"). IANA management of this range of
the Message Type Name Space is unnecessary.
- Message Types 0x3F00 - 0x3FFF. Message types in this range are
reserved for Experimental extensions and are the responsibility
of the individual experimenters (see Sections "LDP Experimental
Extensions" and "Experiment ID Name Space"). IANA management
of this range of the Message Type Name Space is unnecessary;
however, IANA is responsible for managing part of the
Experiment ID Name Space (see below).
4.2. TLV Type Name Space
LDP divides the name space for TLV types into three ranges. The
following are the guidelines for managing these ranges:
- TLV Types 0x0000 - 0x3DFF. TLV types in this range are part of
the LDP base protocol. Following the policies outlined in
[IANA], TLV types in this range are allocated through an IETF
Consensus action.
- TLV Types 0x3E00 - 0x3EFF. TLV types in this range are
reserved for Vendor-Private extensions and are the
responsibility of the individual vendors (see Section "LDP
Vendor-Private TLVs"). IANA management of this range of the
TLV Type Name Space is unnecessary.
- TLV Types 0x3F00 - 0x3FFF. TLV types in this range are
reserved for Experimental extensions and are the responsibility
of the individual experimenters (see Sections "LDP Experimental
Extensions" and "Experiment ID Name Space"). IANA management
of this range of the TLV Name Space is unnecessary; however,
IANA is responsible for managing part of the Experiment ID Name
Space (see below).
4.3. FEC Type Name Space
The range for FEC types is 0 - 255.
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RFC 5036 LDP Specification October 2007
Following the policies outlined in [IANA], FEC types in the range 0 -
127 are allocated through an IETF Consensus action, types in the
range 128 - 191 are allocated as First Come First Served, and types
in the range 192 - 255 are reserved for Private Use.
4.4. Status Code Name Space
The range for Status Codes is 0x00000000 - 0x3FFFFFFF.
Following the policies outlined in [IANA], Status Codes in the range
0x00000000 - 0x1FFFFFFF are allocated through an IETF Consensus
action, codes in the range 0x20000000 - 0x3EFFFFFF are allocated as
First Come First Served, and codes in the range 0x3F000000 -
0x3FFFFFFF are reserved for Private Use.
4.5. Experiment ID Name Space
The range for Experiment IDs is 0x00000000 - 0xffffffff.
Following the policies outlined in [IANA], Experiment IDs in the
range 0x00000000 - 0xefffffff are allocated as First Come First
Served and Experiment IDs in the range 0xf0000000 - 0xffffffff are
reserved for Private Use.
5. Security Considerations
This section identifies threats to which LDP may be vulnerable and
discusses means by which those threats might be mitigated.
5.1. Spoofing
There are two types of LDP communication that could be the target of
a spoofing attack.
1. Discovery exchanges carried by UDP
LSRs indicate their willingness to establish and maintain LDP
sessions by periodically sending Hello messages. Receipt of a
Hello serves to create a new "Hello adjacency", if one does not
already exist, or to refresh an existing one. Spoofing a Hello
packet for an existing adjacency can cause the adjacency to time
out and that can result in termination of the associated session.
This can occur when the spoofed Hello specifies a small Hold Time,
causing the receiver to expect Hellos within this interval, while
the true neighbor continues sending Hellos at the lower,
previously agreed to, frequency.
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RFC 5036 LDP Specification October 2007
LSRs directly connected at the link level exchange Basic Hello
messages over the link. The threat of spoofed Basic Hellos can be
reduced by:
o Accepting Basic Hellos only on interfaces to which LSRs that
can be trusted are directly connected.
o Ignoring Basic Hellos not addressed to the All Routers on
this Subnet multicast group.
LSRs not directly connected at the link level may use Extended
Hello messages to indicate willingness to establish an LDP
session. An LSR can reduce the threat of spoofed Extended Hellos
by filtering them and accepting only those originating at sources
permitted by an access list.
2. Session communication carried by TCP
LDP specifies use of the TCP MD5 Signature Option to provide for
the authenticity and integrity of session messages.
[RFC2385] asserts that MD5 authentication is now considered by
some to be too weak for this application. It also points out that
a similar TCP option with a stronger hashing algorithm (it cites
SHA-1 as an example) could be deployed. To our knowledge, no such
TCP option has been defined and deployed. However, we note that
LDP can use whatever TCP message digest techniques are available,
and when one stronger than MD5 is specified and implemented,
upgrading LDP to use it would be relatively straightforward.
5.2. Privacy
LDP provides no mechanism for protecting the privacy of label
distribution.
The security requirements of label distribution protocols are
essentially identical to those of the protocols that distribute
routing information. By providing a mechanism to ensure the
authenticity and integrity of its messages, LDP provides a level of
security that is at least as good as, though no better than, that
which can be provided by the routing protocols themselves. The more
general issue of whether privacy should be required for routing
protocols is beyond the scope of this document.
One might argue that label distribution requires privacy to address
the threat of label spoofing. However, that privacy would not
protect against label spoofing attacks since data packets carry
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RFC 5036 LDP Specification October 2007
labels in the clear. Furthermore, label spoofing attacks can be made
without knowledge of the FEC bound to a label.
To avoid label spoofing attacks, it is necessary to ensure that
labeled data packets are labeled by trusted LSRs and that the labels
placed on the packets are properly learned by the labeling LSRs.
5.3. Denial of Service
LDP provides two potential targets for Denial of Service (DoS)
attacks:
1. Well-known UDP Port for LDP Discovery
An LSR administrator can address the threat of DoS attacks via
Basic Hellos by ensuring that the LSR is directly connected only
to peers that can be trusted to not initiate such an attack.
Interfaces to peers interior to the administrator's domain should
not represent a threat since interior peers are under the
administrator's control. Interfaces to peers exterior to the
domain represent a potential threat since exterior peers are not.
An administrator can reduce that threat by connecting the LSR only
to exterior peers that can be trusted to not initiate a Basic
Hello attack.
DoS attacks via Extended Hellos are potentially a more serious
threat. This threat can be addressed by filtering Extended Hellos
using access lists that define addresses with which Extended
Discovery is permitted. However, performing the filtering
requires LSR resource.
In an environment where a trusted MPLS cloud can be identified,
LSRs at the edge of the cloud can be used to protect interior LSRs
against DoS attacks via Extended Hellos by filtering out Extended
Hellos originating outside of the trusted MPLS cloud, accepting
only those originating at addresses permitted by access lists.
This filtering protects LSRs in the interior of the cloud but
consumes resources at the edges.
2. Well-known TCP port for LDP Session Establishment
Like other control plane protocols that use TCP, LDP may be the
target of DoS attacks, such as SYN attacks. LDP is no more or
less vulnerable to such attacks than other control plane protocols
that use TCP.
The threat of such attacks can be mitigated somewhat by the
following:
Andersson, et al. Standards Track [Page 88]

RFC 5036 LDP Specification October 2007
o An LSR SHOULD avoid promiscuous TCP listens for LDP session
establishment. It SHOULD use only listens that are specific
to discovered peers. This enables it to drop attack packets
early in their processing since they are less likely to
match existing or in-progress connections.
o The use of the MD5 option helps somewhat since it prevents a
SYN from being accepted unless the MD5 segment checksum is
valid. However, the receiver must compute the checksum
before it can decide to discard an otherwise acceptable SYN
segment.
o The use of access list mechanisms applied at the boundary of
the MPLS cloud in a manner similar to that suggested above
for Extended Hellos can protect the interior against attacks
originating from outside the cloud.
6. Areas for Future Study
The following topics not addressed in this version of LDP are
possible areas for future study:
- Section 2.16 of the MPLS architecture [RFC3031] requires that
the initial label distribution protocol negotiation between
peer LSRs enable each LSR to determine whether its peer is
capable of popping the label stack. This version of LDP
assumes that LSRs support label popping for all link types
except ATM and Frame Relay. A future version may specify means
to make this determination part of the session initiation
negotiation.
- LDP support for CoS (Class of Service) is not specified in this
version. CoS support may be addressed in a future version.
- LDP support for multicast is not specified in this version.
Multicast support may be addressed in a future version.
- LDP support for multipath label switching is not specified in
this version. Multipath support may be addressed in a future
version.
- LDP support for signaling the maximum transmission unit is not
specified in this version. It is discussed in the experimental
document [LDP-MTU].
- The current specification does not address basic peer discovery
on Non-Broadcast Multi-Access (NBMA) media. The solution
available in the current specification is to use extended peer
Andersson, et al. Standards Track [Page 89]

RFC 5036 LDP Specification October 2007
discovery in such setups. The issue of defining a mechanism
semantically similar to Basic Discovery (1 hop limit, bind the
hello adjacency to an interface) that uses preconfigured
neighbor addresses is left for further study.
- The current specification does not support shutting down an
adjacency. The motivation for doing it and the mechanisms for
achieving it are left for further study.
- The current specification does not include a method for
securing Hello messages, to detect spoofing of Hellos. The
scenarios where this is necessary, as well as the mechanism for
achieving it are left for future study.
- The current specification does not have the ability to detect a
stateless fast control plane restart. The method for achieving
this, possibly through an "incarnation/instance" number carried
in the Hello message, is left for future study.
- The current specification does not support an "end of LIB"
message, analogous to BGP's "end of RIB" message that an LDP
LSR (operating in DU mode) would use following session
establishment. The discussion on the need for such a mechanism
and its implementation is left for future study.
- The current specification does not deal with situations where
different LSRs advertise the same address. Such situations
typically occur as the result of configuration errors, and the
goal in this case is to provide the LSRs advertising the same
address with enough information to enable operators to take
corrective action. The specification of this mechanism is left
for a separate document.
7. Changes from RFC 3036
Here is a list of changes from RFC 3036
1. Removed the Host Address FEC and references to it, since it is
not used by any implementation.
2. Split the reference list into normative and informative
references
3. Removed "MPLS using ATM VP Switching" from the list of
normative references, and references to it.
4. Removed reference to RFC 1700 and replaced it with a link to
http://www.iana.org/assignments/address-family-numbers.
Andersson, et al. Standards Track [Page 90]

RFC 5036 LDP Specification October 2007
5. Removed reference to RFC 1771 and replaced it with a reference
to RFC 4271.
6. Clarified the use of the F-bit.
7. Added option to allow split horizon when doing Ordered
Control.
8. Clarified the processing of messages with the U-bit set during
the session initialization procedures
9. Clarified the processing of the E-bit during session
initialization procedures.
10. Added text explaining that the Shutdown message in the state
transition diagram is implemented as a notification message
with a Status TLV indicating a fatal error.
11. Added case for TLV length too short in the specification for
handling malformed TLVs.
12. Explained the security threat posed by hello spoofing.
13. Added reference to 4271 and 4278 and text for standards
maturity variance with regards to the MD5 option.
14. Added text from 3031 explaining the handling of implicit NULL
label.
15. Included the encoding of DLCIs to remove normative reference
to 3034.
16. Moved references to 3031, 3032, and 3034 to informative.
17. In the section describing handling of unknown TLV, removed
reference to inexistent section (errata in original document).
18. Added text clarifying how to achieve interoperability when
sending vendor-private TLVs and messages.
19. In the "receive label request" procedures, if a loop is
detected, changed the procedure to send a notification before
aborting the rest of the processing.
20. In "receive label release" procedures, clarified the behavior
for merge-capable LSRs.
Andersson, et al. Standards Track [Page 91]

RFC 5036 LDP Specification October 2007
21. In "receive label release" procedures, clarified the behavior
for receipt of an unknown FEC.
22. In note 4 of "Detect Change in FEC Next Hop", modified the
text to reference the correct set of conditions for sending a
label request procedure (typo in the original document).
23. In the procedures for "LSR decides to no longer label switch a
FEC", clarified the fact that the label must not be reused
until a label release is received.
24. In the routine "Prepare_Label_Mapping_Attributes", added a
note regarding the treatment of unknown TLVs according to
their U and F-bits.
25. In the Address message processing procedures, clarified the
behavior for the case where an LSR receives re-advertisement
of an address previously advertised it, or withdrawal of an
address from an LSR that has not previously advertised that
address.
26. In the routine "Receive Label Mapping", clarified the meaning
of PrevAdvLabel when no label advertisement message has been
sent previously.
27. In the "Receive Label Mapping" procedures, if a loop is
detected, modified the procedure to send a notification before
aborting the rest of the processing.
28. In the "Receive Label Mapping" procedures, corrected step
LMp.10 to handle label mapping messages for additional (non-
merged) LSPs for the FEC.
29. In the "Receive Label Mapping" procedures, clarified behavior
when receiving a duplicate label for the same FEC.
30. In the routine "Receive Label Abort Request", clarified the
behavior for non-merging LSRs.
31. Added the following items to the section discussing areas for
future study:
o extensions for communicating the maximum transmission unit
o basic peer discovery on NBMA media
o option of shutting down an adjacency
o mechanisms for securing Hello messages
o detection of a stateless fast control plane restart
o support of "end of LIB" message
Andersson, et al. Standards Track [Page 92]

RFC 5036 LDP Specification October 2007
o mechanisms for dealing with the case where different LSRs
advertise the same address
8. Acknowledgments
This document is produced as part of advancing the LDP specification
to draft standard status. This document was originally published as
RFC 3036 in January 2001. It was produced by the MPLS Working Group
of the IETF and was jointly authored by Loa Andersson, Paul Doolan,
Nancy Feldman, Andre Fredette, and Bob Thomas.
The ideas and text in RFC 3036 were collected from a number of
sources. We would like to thank Rick Boivie, Ross Callon, Alex
Conta, Eric Gray, Yoshihiro Ohba, Eric Rosen, Bernard Suter, Yakov
Rekhter, and Arun Viswanathan for their input for RFC 3036.
The editors would like to thank Eric Gray, David Black, and Sam
Hartman for their input to and review of the current document.
In addition, the editors would like to thank the members of the MPLS
Working Group for their ideas and the support they have given to this
document, and in particular, to Eric Rosen, Luca Martini, Markus
Jork, Mark Duffy, Vach Kompella, Kishore Tiruveedhula, Rama
Ramakrishnan, Nick Weeds, Adrian Farrel, and Andy Malis.
9. References9.1. Normative References
[IANA] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26, RFC2434, October 1998.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC1321, April 1992.
[ASSIGNED_AF] http://www.iana.org/assignments/address-family-numbers
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP
MD5 Signature Option", RFC 2385, August 1998.
[RFC3035] Davie, B., Lawrence, J., McCloghrie, K., Rosen, E.,
Swallow, G., Rekhter, Y., and P. Doolan, "MPLS using
LDP and ATM VC Switching", RFC 3035, January 2001.
Andersson, et al. Standards Track [Page 93]

RFC 5036 LDP Specification October 2007
. Downstream Unsolicited Ordered Control, called
PushConditional in [RFC3031].
. Downstream On Demand Independent Control, called
PulledUnconditional in [RFC3031].
. Downstream On Demand Ordered Control, called
PulledConditional in [RFC3031].
- Label Withdrawal procedure, which is performed by a downstream
LSR to determine when to withdraw a FEC label mapping
previously distributed to LDP peers. The architecture defines
a single Label Withdrawal procedure. Whenever an LSR breaks
the binding between a label and a FEC, it MUST withdraw the FEC
label mapping from all LDP peers to which it has previously
sent the mapping.
- Label Request procedure, which is performed by an upstream LSR
to determine when to explicitly request that a downstream LSR
bind a label to a FEC and send it the corresponding label
mapping. The architecture defines three Label Request
procedures:
. Request Never. The LSR never requests a label.
. Request When Needed. The LSR requests a label whenever it
needs one.
. Request On Request. This procedure is used by non-label
merging LSRs. The LSR requests a label when it receives a
request for one, in addition to whenever it needs one.
- Label Release procedure, which is performed by an upstream LSR
to determine when to release a previously received label
mapping for a FEC. The architecture defines two Label Release
procedures:
. Conservative Label retention, called ReleaseOnChange in
[RFC3031].
. Liberal Label retention, called NoReleaseOnChange in
[RFC3031].
- Label Use procedure, which is performed by an LSR to determine
when to start using a FEC label for forwarding/switching. The
architecture defines three Label Use procedures:
Andersson, et al. Standards Track [Page 96]

RFC 5036 LDP Specification October 2007
. Use Immediate. The LSR immediately uses a label received
from a FEC next hop for forwarding/switching.
. Use If Loop Free. The LSR uses a FEC label received from a
FEC next hop for forwarding/switching only if it has
determined that by doing so it will not cause a forwarding
loop.
. Use If Loop Not Detected. This procedure is the same as Use
Immediate unless the LSR has detected a loop in the FEC LSP.
Use of the FEC label for forwarding/switching will continue
until the next hop for the FEC changes or the loop is no
longer detected.
This version of LDP does not include a loop prevention
mechanism; therefore, the procedures below do not make use of
the Use If Loop Free procedure.
- Label No Route procedure (called the NotAvailable procedure in
[RFC3031]), which is performed by an upstream LSR to determine
how to respond to a No Route notification from a downstream LSR
in response to a request for a FEC label mapping. The
architecture specification defines two Label No Route
procedures:
. Request Retry. The LSR should issue the label request at a
later time.
. No Request Retry. The LSR should assume that the downstream
LSR will provide a label mapping when the downstream LSR has
a next hop, and it should not reissue the request.
A.1. Handling Label Distribution Events
This section defines LDP label distribution procedures by specifying
an algorithm for each label distribution event. The requirement on
an LDP implementation is that its event handling must have the effect
specified by the algorithms. That is, an implementation need not
follow exactly the steps specified by the algorithms as long as the
effect is identical.
The algorithms for handling label distribution events share common
actions. The specifications below package these common actions into
procedure units. Specifications for these common procedures are in
their own Section, "Common Label Distribution Procedures", which
follows this.
Andersson, et al. Standards Track [Page 97]

RFC 5036 LDP Specification October 2007
An implementation would use data structures to store information
about protocol activity. This appendix specifies the information to
be stored in sufficient detail to describe the algorithms, and
assumes the ability to retrieve the information as needed. It does
not specify the details of the data structures.
A.1.1. Receive Label Request
Summary:
The response by an LSR to receipt of a FEC label request from an
LDP peer may involve one or more of the following actions:
- Transmission of a notification message to the requesting LSR
indicating why a label mapping for the FEC cannot be provided;
- Transmission of a FEC label mapping to the requesting LSR;
- Transmission of a FEC label request to the FEC next hop;
- Installation of labels for forwarding/switching use by the LSR.
Context:
- LSR. The LSR handling the event.
- MsgSource. The LDP peer that sent the message.
- FEC. The FEC specified in the message.
- RAttributes. Attributes received with the message, e.g., Hop
Count, Path Vector.
- SAttributes. Attributes to be included in the Label Request
message, if any, propagated to FEC Next Hop.
- StoredHopCount. The hop count, if any, previously recorded for
the FEC.
Algorithm:
LRq.1 Execute procedure Check_Received_Attributes (MsgSource,
LabelRequest, RAttributes).
If Loop Detected, goto LRq.4.
LRq.2 Is there a Next Hop for FEC?
If not, goto LRq.5.
Andersson, et al. Standards Track [Page 98]

RFC 5036 LDP Specification October 2007
Notes:
1. In the case where MsgSource is a non-label merging LSR, it
will send a label request for each upstream LDP peer that
has requested a label for FEC from it. The LSR must be able
to distinguish such requests from a non-label merging
MsgSource from duplicate label requests.
The LSR uses the message ID of received Label Request
messages to detect duplicate requests. This means that an
LSR (the upstream peer) may not reuse the message ID used
for a Label Request until the Label Request transaction has
completed.
2. When an LSR sends a label request to a peer, it records that
the request has been sent and marks it as outstanding. As
long as the request is marked outstanding, the LSR SHOULD
NOT send another request for the same label to the peer.
Such a second request would be a duplicate. The
Send_Label_Request procedure described below obeys this
rule.
A duplicate label request is considered a protocol error and
SHOULD be dropped by the receiving LSR (perhaps with a
suitable notification returned to MsgSource).
3. If the LSR is not merge-capable, this test will fail.
4. The Send_Label procedure may fail due to lack of label
resources, in which case the LSR SHOULD NOT perform the
Label Use procedure.
A.1.2. Receive Label Mapping
Summary:
The response by an LSR to receipt of a FEC label mapping from an
LDP peer may involve one or more of the following actions:
- Transmission of a Label Release message for the FEC label to
the LDP peer;
- Transmission of Label Mapping messages for the FEC to one or
more LDP peers;
- Installation of the newly learned label for
forwarding/switching use by the LSR.
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RFC 5036 LDP Specification October 2007
Context:
- LSR. The LSR handling the event.
- MsgSource. The LDP peer that sent the message.
- FEC. The FEC specified in the message.
- Label. The label specified in the message.
- PrevAdvLabel. The label for the FEC, if any, previously
advertised to an upstream peer. Assuming no label was
previously advertised, this is the same label as the one in the
Label Mapping message being processed.
- StoredHopCount. The hop count previously recorded for the FEC.
- RAttributes. Attributes received with the message, e.g., Hop
Count, Path Vector.
- SAttributes to be included in the Label Mapping message, if
any, propagated to upstream peers.
Algorithm:
LMp.1 Does the received label mapping match an outstanding label
request for FEC previously sent to MsgSource? If not,
goto LMp.3.
LMp.2 Delete record of outstanding FEC label request.
LMp.3 Execute procedure Check_Received_Attributes (MsgSource,
LabelMapping, RAttributes).
If No Loop Detected, goto LMp.9.
LMp.4 Does the LSR have a previously received label mapping for
FEC from MsgSource? (See Note 1.)
If not, goto LMp.8. (See Note 2.)
LMp.5 Does the label previously received from MsgSource match
Label (i.e., the label received in the message)? (See
Note 3.)
If not, goto LMp.8. (See Note 4.)
LMp.6 Delete matching label mapping for FEC previously
received from MsgSource.
LMp.7 Remove Label from forwarding/switching use. (See Note 5.)
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RFC 5036 LDP Specification October 2007
LMp.8 Execute procedure Send_Message (MsgSource, Label Release,
FEC, Label, Loop Detected Status code). Goto LMp.33.
LMp.9 Does LSR have a previously received label mapping for FEC
from MsgSource for the LSP in question? (See Note 6.)
If not, goto LMp.11.
LMp.10 Does the label previously received from MsgSource match
Label (i.e., the label received in the message)? (See
Note 3.)
OR
Is the received label mapping in response to a previously
outstanding label request sent to MsgSource? (See Note
12.)
If so, goto LMp.11.
LMp.10a Is LSR operating in Downstream Unsolicited mode? If so,
delete the label mapping for the label previously received
from MsgSource and remove it from forwarding/switching
use.
Execute procedure Send_Message (MsgSource, Label Release,
FEC, label previously received from MsgSource).
LMp.11 Determine the Next Hop for FEC.
LMp.12 Is MsgSource the Next Hop for FEC?
If so, goto LMp.14.
LMp.13 Perform LSR Label Release procedure:
For Conservative Label retention:
1. Goto LMp.32.
For Liberal Label retention:
1. Record label mapping for FEC with Label and
RAttributes has been received from MsgSource.
Goto LMp.33.
LMp.14 Is LSR an ingress for FEC?
If not, goto LMp.16.
LMp.15 Install Label for forwarding/switching use.
LMp.16 Record label mapping for FEC with Label and RAttributes
has been received from MsgSource.
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RFC 5036 LDP Specification October 2007
For Downstream Unsolicited Independent Control OR For
Downstream Unsolicited Ordered Control
1. Execute procedure
Prepare_Label_Mapping_Attributes (Peer, FEC,
RAttributes, SAttributes, IsPropagating,
UnknownHopCount).
2. Execute procedure Send_Label (Peer, FEC, SAttributes).
If the procedure fails, continue iteration for next
Peer at LMp.17.
3. If no pending requests exist for Peer, goto LMp.30.
(See Note 11.)
For Downstream On Demand Independent Control OR For
Downstream On Demand Ordered Control
1. Iterate through Step 5 for each pending label request
for FEC from Peer marked as pending.
2. Execute procedure
Prepare_Label_Mapping_Attributes (Peer, FEC,
RAttributes, SAttributes, IsPropagating,
UnknownHopCount)
3. Execute procedure Send_Label (Peer, FEC, SAttributes).
If the procedure fails, continue iteration for next
Peer at LMp.17.
4. Delete record of pending request.
5. End iteration from Step 1.
6. Goto LMp.30.
LMp.30 Perform LSR Label Use procedure:
For Use Immediate OR For Use If Loop Not Detected
1. Iterate through Step 3 for each label mapping for FEC
previously sent to Peer.
2. Install label received and label sent to Peer for
forwarding/switching use.
3. End iteration from Step 1.
Andersson, et al. Standards Track [Page 105]

RFC 5036 LDP Specification October 2007
4. Goto LMp.31.
LMp.31 End iteration from LMp.17.
Go to LMp.33.
LMp.32 Execute procedure Send_Message (MsgSource, Label Release,
FEC, Label).
LMp.33 DONE.
Notes:
1. If the LSR is merging, there should be at most 1 received
mapping for the FEC for the LSP in question. In the non-
merging case, there could be multiple received mappings for
the FEC for the LSP in question.
2. If the LSR has detected a loop and it has not previously
received a label mapping from MsgSource for the FEC, it simply
releases the label.
3. Does the Label received in the message match any of the 1 or
more label mappings identified in the previous step (LMp.4 or
LMp.9)?
4. An unsolicited mapping with a different label from the same
peer would be an attempt to establish multipath label
switching, which is not supported in this version of LDP.
5. If the Label is not in forwarding/switching use, LMp.7 has no
effect.
6. If the received label mapping message matched an outstanding
label request in LMp.1, then (by definition) the LSR has not
previously received a label mapping for FEC for the LSP in
question. If the LSR is merging upstream labels for the LSP
in question, there should be at most 1 received mapping. In
the non-merging case, there could be multiple received label
mappings for the same FEC, one for each resulting LSP.
7. The LMp.17 iteration includes MsgSource in order to handle the
case where the LSR is operating in Downstream Unsolicited
Ordered Control mode. Ordered Control prevents the LSR from
advertising a label for the FEC until it has received a label
mapping from its next hop (MsgSource) for the FEC.
8. If the LSR is merging the LSP, it may have previously sent
label mappings for the FEC LSP to one or more peers. If the
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RFC 5036 LDP Specification October 2007
LSR is not merging, it may have sent a label mapping for the
LSP in question to at most one LSR.
9. The Loop Detection Path Vector attribute is considered in this
check. If the received RAttributes include a Path Vector and
no Path Vector had been previously sent to the Peer, or if the
received Path Vector is inconsistent with the Path Vector
previously sent to the Peer, then the attributes are
considered to be inconsistent. Note that an LSR is not
required to store a received Path Vector after it propagates
the Path Vector in a mapping message. If an LSR does not
store the Path Vector, it has no way to check the consistency
of a newly received Path Vector. This means that whenever
such an LSR receives a mapping message carrying a Path Vector
it must always propagate the Path Vector.
10. LMp.22 through LMp.27 deal with a situation that can arise
when the LSR is using independent control and it receives a
mapping from the downstream peer after it has sent a mapping
to an upstream peer. In this situation, the LSR needs to
propagate any changed attributes, such as Hop Count, upstream.
If Loop Detection is configured on, the propagated attributes
must include the Path Vector.
11. An LSR operating in Downstream Unsolicited mode MUST process
any Label Request messages it receives. If there are pending
label requests, fall through into the Downstream on Demand
procedures in order to satisfy the pending requests.
12. As determined by step LMp.1.
13. An LSR operating in Ordered Control mode may choose to skip at
this stage the peer from which it received the advertisement
that caused it to generate the label-map message. Doing so
will in effect provide a form of split-horizon.
A.1.3. Receive Label Abort Request
Summary:
When an LSR receives a Label Abort Request message from a peer, it
checks whether it has already responded to the label request in
question. If it has, it silently ignores the message. If it has
not, it sends the peer a Label Request Aborted Notification. In
addition, if it has a label request outstanding for the LSP in
question to a downstream peer, it sends a Label Abort Request to
the downstream peer to abort the LSP.
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RFC 5036 LDP Specification October 2007
Context:
- LSR. The LSR handling the event.
- MsgSource. The LDP peer that sent the message.
- FEC. The FEC specified in the message.
- RequestMessageID. The message ID of the label request message
to be aborted.
- Next Hop. The next hop for the FEC.
Algorithm:
LAbR.1 Does the message match a previously received Label Request
message from MsgSource? (See Note 1.)
If not, goto LAbR.12.
LAbR.2 Has LSR responded to the previously received label
request?
If so, goto LAbR.12.
LAbR.3 Execute procedure Send_Message (MsgSource, Notification,
Label Request Aborted, TLV), where TLV is the Label
Request Message ID TLV received in the Label Abort Request
message.
LAbR.4 Does LSR have a Label Request message outstanding for FEC?
If so, goto LAbR.7.
LAbR.5 Does LSR have a label mapping for FEC? If not, goto
LAbR.11.
LAbR.6 Generate Event: Received Label Release message for FEC
from MsgSource. (See Note 2.)
Goto LAbR.11.
LAbR.7 Is LSR merging the LSP for FEC?
If not, goto LAbR.9.
LAbR.8 Are there outstanding label requests for this FEC?
If so, goto LAbR.11.
LAbR.9 Execute procedure Send_Message (Next Hop, Label Abort
Request, FEC, TLV), where TLV is a Label Request message
ID TLV containing the Message ID used by the LSR in the
outstanding Label Request message.
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RFC 5036 LDP Specification October 2007
LAbR.10 Record that a label abort request for FEC is pending.
LAbR.11 Delete record of label request for FEC from MsgSource.
LAbR.12 DONE.
Notes:
1. LSR uses FEC and the Label Request message ID TLV carried by
the label abort request to locate its record (if any) for the
previously received label request from MsgSource.
2. If LSR has received a label mapping from NextHop, it should
behave as if it had advertised a label mapping to MsgSource and
MsgSource has released it.
A.1.4. Receive Label Release
Summary:
When an LSR receives a Label Release message for a FEC from a
peer, it checks whether other peers hold the released label. If
none do, the LSR removes the label from forwarding/switching use,
if it has not already done so, and if the LSR holds a label
mapping from the FEC next hop, it releases the label mapping.
Context:
- LSR. The LSR handling the event.
- MsgSource. The LDP peer that sent the message.
- Label. The label specified in the message.
- FEC. The FEC specified in the message.
Algorithm:
LRl.1 Does FEC match a known FEC? If not, goto LRl.14.
LRl.2 Remove MsgSource from record of peers that hold Label for
FEC. (See Note 1.)
LRl.3 Does message match an outstanding label withdraw for FEC
previously sent to MsgSource?
If not, goto LRl.5
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RFC 5036 LDP Specification October 2007
LRl.4 Delete record of outstanding label withdraw for FEC
previously sent to MsgSource.
LRl.5 Is LSR merging labels for this FEC? If not, goto LRl.7.
(See Note 2.)
LRl.6 Does LSR have outstanding label advertisements for this
FEC?
If so, goto LRl.11.
LRl.7 Is LSR egress for the FEC?
If so, goto LRl.11.
LRl.8 Is there a Next Hop for FEC? AND Does LSR have a
previously received label mapping for FEC from Next Hop?
If not, goto LRl.11.
LRl.9 Is LSR configured to propagate releases?
If not, goto LRl.11. (See Note 3.)
LRl.10 Execute procedure Send_Message (Next Hop, Label Release,
FEC, Label from Next Hop).
LRl.11 Remove Label from forwarding/switching use for traffic
from MsgSource.
LRl.12 Do any peers still hold Label for FEC?
If so, goto LRl.14.
LRl.13 Free the Label.
LRl.14 DONE.
Notes:
1. If LSR is using Downstream Unsolicited label distribution, it
SHOULD NOT re-advertise a label mapping for FEC to MsgSource
until MsgSource requests it.
2. LRl.5 through LRl.9 deal with determining whether where the LSR
should propagate the Label Release to a downstream peer
(LRl.9).
3. If LRl.9 is reached, no upstream LSR holds a label for the FEC,
and the LSR holds a label for the FEC from the FEC Next Hop.
The LSR could propagate the Label Release to the Next Hop. By
propagating the Label Release, the LSR releases a potentially
scarce label resource. In doing so, it also increases the
Andersson, et al. Standards Track [Page 110]

RFC 5036 LDP Specification October 2007
latency for re-establishing the LSP should MsgSource or some
other upstream LSR send it a new Label Request for FEC.
Whether or not to propagate the release is not a protocol
issue. Label distribution will operate properly whether or not
the release is propagated. The decision to propagate or not
should take into consideration factors such as, whether labels
are a scarce resource in the operating environment, the
importance of keeping LSP setup latency low by keeping the
amount of signaling required small, and whether LSP setup is
ingress-controlled or egress-controlled in the operating
environment.
A.1.5. Receive Label Withdraw
Summary:
When an LSR receives a Label Withdraw message for a FEC from an
LDP peer, it responds with a Label Release message and it removes
the label from any forwarding/switching use. If Ordered Control
is in use, the LSR sends a Label Withdraw message to each LDP peer
to which it had previously sent a label mapping for the FEC. If
the LSR is using Downstream on Demand label advertisement with
independent control, it then acts as if it had just recognized the
FEC.
Context:
- LSR. The LSR handling the event.
- MsgSource. The LDP peer that sent the message.
- Label. The label specified in the message.
- FEC. The FEC specified in the message.
Algorithm:
LWd.1 Remove Label from forwarding/switching use. (See Note 1.)
LWd.2 Execute procedure Send_Message (MsgSource, Label Release,
FEC, Label).
LWd.3 Has LSR previously received and retained a matching label
mapping for FEC from MsgSource?
If not, goto LWd.13.
Andersson, et al. Standards Track [Page 111]

RFC 5036 LDP Specification October 2007
LWd.4 Delete matching label mapping for FEC previously received
from MsgSource.
LWd.5 Is LSR using Ordered Control?
If so, goto LWd.8.
LWd.6 Is MsgSource using Downstream On Demand label
advertisement?
If not, goto LWd.13.
LWd.7 Generate Event: Recognize New FEC for FEC. Goto LWd.13.
(See Note 2.)
LWd.8 Iterate through LWd.12 for each Peer, other than
MsgSource.
LWd.9 Has LSR previously sent a label mapping for FEC to Peer?
If not, continue iteration for next Peer at LWd.8.
LWd.10 Does the label previously sent to Peer "map" to the
withdrawn Label?
If not, continue iteration for next Peer at LWd.8. (See
Note 3.)
LWd.11 Execute procedure Send_Label_Withdraw (Peer, FEC, Label
previously sent to Peer).
LWd.12 End iteration from LWd.8.
LWd.13 DONE.
Notes:
1. If the Label is not in forwarding/switching use, LWd.1 has no
effect.
2. LWd.7 handles the case where the LSR is using Downstream On
Demand label distribution with independent control. In this
situation, the LSR should send a label request to the FEC next
hop as if it had just recognized the FEC.
3. LWd.10 handles both label merging (one or more incoming labels
map to the same outgoing label) and no label merging (one label
maps to the outgoing label) cases.
Andersson, et al. Standards Track [Page 112]

RFC 5036 LDP Specification October 2007A.1.6. Recognize New FEC
Summary:
The response by an LSR to learning a new FEC via the routing table
may involve one or more of the following actions:
- Transmission of label mappings for the FEC to one or more LDP
peers;
- Transmission of a label request for the FEC to the FEC next
hop;
- Any of the actions that can occur when the LSR receives a label
mapping for the FEC from the FEC next hop.
Context:
- LSR. The LSR handling the event.
- FEC. The newly recognized FEC.
- Next Hop. The next hop for the FEC.
- InitAttributes. Attributes to be associated with the new FEC.
(See Note 1.)
- SAttributes. Attributes to be included in Label Mapping or
Label Request messages, if any, sent to peers.
- StoredHopCount. Hop count associated with FEC label mapping,
if any, previously received from Next Hop.
Algorithm:
FEC.1 Perform LSR Label Distribution procedure:
For Downstream Unsolicited Independent Control
1. Iterate through 5 for each Peer.
2. Has LSR previously received and retained a label
mapping for FEC from Next Hop?
If so, set Propagating to IsPropagating.
If not, set Propagating to NotPropagating.
Andersson, et al. Standards Track [Page 113]

RFC 5036 LDP Specification October 2007
For Request When Needed OR
For Request On Request
1. Execute procedure
Prepare_Label_Request_Attributes (Next Hop, FEC,
InitAttributes, SAttributes);
2. Execute procedure Send_Label_Request (Next Hop, FEC,
SAttributes).
Goto FEC.6.
FEC.5 Generate Event: Received Label Mapping from Next Hop.
(See Note 3.)
FEC.6 DONE.
Notes:
1. An example of an attribute that might be part of InitAttributes
is one that specifies desired LSP characteristics, such as
Class of Service (CoS). (Note that while the current version
of LDP does not specify a CoS attribute, LDP extensions may.)
The means by which FEC InitAttributes, if any, are specified is
beyond the scope of LDP. Note that the InitAttributes will not
include a known Hop Count or a Path Vector.
2. An LSR using Downstream On Demand label distribution would send
a label only if it had a previously received label request
marked as pending. The LSR would have no such pending requests
because it responds to any label request for an unknown FEC by
sending the requesting LSR a No Route notification and
discarding the label request; see LRq.3
3. If the LSR has a label for the FEC from the Next Hop, it should
behave as if it had just received the label from the Next Hop.
This occurs in the case of Liberal Label retention mode.
A.1.7. Detect Change in FEC Next Hop
Summary:
The response by an LSR to a change in the next hop for a FEC may
involve one or more of the following actions:
- Removal of the label from the FEC's old next hop from
forwarding/switching use;
Andersson, et al. Standards Track [Page 115]

RFC 5036 LDP Specification October 2007
- Transmission of label mapping messages for the FEC to one or
more LDP peers;
- Transmission of a label request to the FEC's new next hop;
- Any of the actions that can occur when the LSR receives a label
mapping from the FEC's new next hop.
Context:
- LSR. The LSR handling the event.
- FEC. The FEC whose next hop changed.
- New Next Hop. The current next hop for the FEC.
- Old Next Hop. The previous next hop for the FEC.
- OldLabel. Label, if any, previously received from Old Next
Hop.
- CurAttributes. The attributes, if any, currently associated
with the FEC.
- SAttributes. Attributes to be included in the Label Request
message, if any, sent to New Next Hop.
Algorithm:
NH.1 Has LSR previously received and retained a label mapping
for FEC from Old Next Hop? If not, goto NH.6.
NH.2 Remove label from forwarding/switching use. (See Note 1.)
NH.3 Is LSR using Liberal Label retention?
If so, goto NH.6.
NH.4 Execute procedure Send_Message (Old Next Hop, Label
Release, OldLabel).
NH.5 Delete label mapping for FEC previously received from Old
Next Hop.
NH.6 Does LSR have a label request pending with Old Next Hop?
If not, goto NH.10.
NH.7 Is LSR using Conservative Label retention?
If not, goto NH.10.
Andersson, et al. Standards Track [Page 116]

RFC 5036 LDP Specification October 2007
NH.8 Execute procedure Send_Message (Old Next Hop, Label Abort
Request, FEC, TLV), where TLV is a Label Request Message ID
TLV that carries the message ID of the pending label
request.
NH.9 Record that a label abort request is pending for FEC with
Old Next Hop.
NH.10 Is there a New Next Hop for FEC?
If not, goto NH.16.
NH.11 Has LSR previously received and retained a label mapping
for FEC from New Next Hop?
If not, goto NH.13.
NH.12 Generate Event: Received Label Mapping from New Next Hop.
Goto NH.20. (See Note 2.)
NH.13 Is LSR using Downstream on Demand advertisement? OR Is Next
Hop using Downstream on Demand advertisement? OR Is LSR
using Conservative Label retention? (See Note 3.)
If so, goto NH.14.
If not, goto NH.20.
NH.14 Execute procedure Prepare_Label_Request_Attributes (Next
Hop, FEC, CurAttributes, SAttributes).
NH.15 Execute procedure Send_Label_Request (New Next Hop, FEC,
SAttributes). (See Note 4.)
Goto NH.20.
NH.16 Iterate through NH.19 for each Peer.
NH.17 Has LSR previously sent a label mapping for FEC to Peer?
If not, continue iteration for next Peer at NH.16.
NH.18 Execute procedure Send_Label_Withdraw (Peer, FEC, Label
previously sent to Peer).
NH.19 End iteration from NH.16.
NH.20 DONE.
Notes:
1. If the Label is not in forwarding/switching use, NH.2 has no
effect.
Andersson, et al. Standards Track [Page 117]

RFC 5036 LDP Specification October 2007
2. If the LSR has a label for the FEC from the New Next Hop, it
should behave as if it had just received the label from the New
Next Hop.
3. The purpose of the check on label retention mode is to avoid a
race with steps LMp.12-LMp.13 of the procedure for handling a
Label Mapping message where the LSR operating in Conservative
Label retention mode may have released a label mapping received
from the New Next Hop before it detected that the FEC next hop
had changed.
4. Regardless of the Label Request procedure in use by the LSR, it
MUST send a label request if the conditions in NH.13 hold.
Therefore, it executes the Send_Label_Request procedure
directly rather than perform the LSR Label Request procedure.
A.1.8. Receive Notification / Label Request Aborted
Summary:
When an LSR receives a Label Request Aborted notification from an
LDP peer, it records that the corresponding label request
transaction, if any, has completed.
Context:
- LSR. The LSR handling the event.
- FEC. The FEC for which a label was requested.
- RequestMessageID. The message ID of the label request message
to be aborted.
- MsgSource. The LDP peer that sent the Notification message.
Algorithm:
LRqA.1 Does the notification correspond to an outstanding label
request abort for FEC? (See Note 1.)
If not, goto LRqA.3.
LRqA.2 Record that the label request for FEC has been aborted.
LRqA.3 DONE.
Andersson, et al. Standards Track [Page 118]

RFC 5036 LDP Specification October 2007
Note:
1. The LSR uses the FEC and RequestMessageID to locate its record,
if any, of the outstanding label request abort.
A.1.9. Receive Notification / No Label Resources
Summary:
When an LSR receives a No Label Resources notification from an LDP
peer, it stops sending label request messages to the peer until it
receives a Label Resources Available Notification from the peer.
Context:
- LSR. The LSR handling the event.
- FEC. The FEC for which a label was requested.
- MsgSource. The LDP peer that sent the Notification message.
Algorithm:
NoRes.1 Delete record of outstanding label request for FEC sent to
MsgSource.
NoRes.2 Record that label mapping for FEC from MsgSource is needed
but that no label resources are available.
NoRes.3 Set status record indicating it is not OK to send label
requests to MsgSource.
NoRes.4 DONE.
A.1.10. Receive Notification / No Route
Summary:
When an LSR receives a No Route notification from an LDP peer in
response to a Label Request message, the Label No Route procedure
in use dictates its response. The LSR either will take no further
action, or it will defer the label request by starting a timer and
send another Label Request message to the peer when the timer
later expires.
Andersson, et al. Standards Track [Page 119]

RFC 5036 LDP Specification October 2007
Context:
- LSR. The LSR handling the event.
- FEC. The FEC for which a label was requested.
- Attributes. The attributes associated with the label request.
- MsgSource. The LDP peer that sent the Notification message.
Algorithm:
NoNH.1 Delete record of outstanding label request for FEC sent to
MsgSource.
NoNH.2 Perform LSR Label No Route procedure.
For Request No Retry
1. Goto NoNH.3.
For Request Retry
1. Record deferred label request for FEC and Attributes
to be sent to MsgSource.
2. Start timeout. Goto NoNH.3.
NoNH.3 DONE.
A.1.11. Receive Notification / Loop Detected
Summary:
When an LSR receives a Loop Detected Status Code from an LDP peer
in response to a Label Request message or a Label Mapping message,
it behaves as if it had received a No Route notification.
Context:
See "Receive Notification / No Route".
Algorithm:
See "Receive Notification / No Route".
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RFC 5036 LDP Specification October 2007
Note:
1. When the Loop Detected notification is in response to a Label
Request message, it arrives in a Status Code TLV in a
Notification message. When it is in response to a Label
Mapping message, it arrives in a Status Code TLV in a Label
Release message.
A.1.12. Receive Notification / Label Resources Available
Summary:
When an LSR receives a Label Resources Available notification from
an LDP peer, it resumes sending label requests to the peer.
Context:
- LSR. The LSR handling the event.
- MsgSource. The LDP peer that sent the Notification message.
- SAttributes. Attributes stored with the postponed Label
Request message.
Algorithm:
Res.1 Set status record indicating it is OK to send label
requests to MsgSource.
Res.2 Iterate through Res.6 for each record of a FEC label
mapping needed from MsgSource for which no label resources
are available.
Res.3 Is MsgSource the next hop for FEC?
If not, goto Res.5.
Res.4 Execute procedure Send_Label_Request (MsgSource, FEC,
SAttributes). If the procedure fails, terminate
iteration.
Res.5 Delete record that no resources are available for a label
mapping for FEC needed from MsgSource.
Res.6 End iteration from Res.2.
Res.7 DONE.
Andersson, et al. Standards Track [Page 121]

RFC 5036 LDP Specification October 2007A.1.13. Detect Local Label Resources Have Become Available
Summary:
After an LSR has sent a No Label Resources notification to an LDP
peer, when label resources later become available it sends a Label
Resources Available notification to each such peer.
Context:
- LSR. The LSR handling the event.
- Attributes. Attributes stored with the postponed Label Mapping
message.
Algorithm:
ResA.1 Iterate through ResA.4 for each Peer to which LSR has
previously sent a No Label Resources notification.
ResA.2 Execute procedure Send_Notification (Peer, Label Resources
Available).
ResA.3 Delete record that No Label Resources notification was
previously sent to Peer.
ResA.4 End iteration from ResA.1.
ResA.5 Iterate through ResA.8 for each record of a label mapping
needed for FEC for Peer but no-label-resources. (See Note
1.)
ResA.6 Execute procedure Send_Label (Peer, FEC, Attributes). If
the procedure fails, terminate iteration.
ResA.7 Clear record of FEC label mapping needed for peer but no-
label-resources.
ResA.8 End iteration from ResA.5
ResA.9 DONE.
Note:
1. Iteration ResA.5 through ResA.8 handles the situation where the
LSR is using Downstream Unsolicited label distribution and was
previously unable to allocate a label for a FEC.
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RFC 5036 LDP Specification October 2007A.1.14. LSR Decides to No Longer Label Switch a FEC
Summary:
An LSR may unilaterally decide to no longer label switch a FEC for
an LDP peer. An LSR that does so MUST send a Label Withdraw
message for the FEC to the peer.
Context:
- Peer. The peer.
- FEC. The FEC.
- PrevAdvLabel. The label for the FEC previously advertised to
the Peer.
Algorithm:
NoLS.1 Execute procedure Send_Label_Withdraw (Peer, FEC,
PrevAdvLabel). (See Note 1.)
NoLS.2 DONE.
Note:
1. The LSR may remove the label from forwarding/switching use as
part of this event or as part of processing the Label Release
from the peer in response to the label withdraw. If the LSR
doesn't wait for the Label Release message from the peer, it
SHOULD NOT reuse the label until it receives the Label Release.
A.1.15. Timeout of Deferred Label Request
Summary:
Label requests are deferred in response to No Route and Loop
Detected notifications. When a deferred FEC label request for a
peer times out, the LSR sends the label request.
Context:
- LSR. The LSR handling the event.
- FEC. The FEC associated with the timeout event.
- Peer. The LDP peer associated with the timeout event.
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RFC 5036 LDP Specification October 2007
- Attributes. Attributes stored with the deferred Label Request
message.
Algorithm:
TO.1 Retrieve the record of the deferred label request.
TO.2 Is Peer the next hop for FEC?
If not, goto TO.4.
TO.3 Execute procedure Send_Label_Request (Peer, FEC).
TO.4 DONE.
A.2. Common Label Distribution Procedures
This section specifies utility procedures used by the algorithms that
handle label distribution events.
A.2.1. Send_Label
Summary:
The Send_Label procedure allocates a label for a FEC for an LDP
peer, if possible, and sends a label mapping for the FEC to the
peer. If the LSR is unable to allocate the label and if it has a
pending label request from the peer, it sends the LDP peer a No
Label Resources notification.
Parameters:
- Peer. The LDP peer to which the label mapping is to be sent.
- FEC. The FEC for which a label mapping is to be sent.
- Attributes. Attributes to be included with the label mapping.
Additional Context:
- LSR. The LSR executing the procedure.
- Label. The label allocated and sent to Peer.
Algorithm:
SL.1 Does LSR have a label to allocate?
If not, goto SL.9.
Andersson, et al. Standards Track [Page 124]

RFC 5036 LDP Specification October 2007
SL.2 Allocate Label and bind it to the FEC.
SL.3 Install Label for forwarding/switching use.
SL.4 Execute procedure Send_Message (Peer, Label Mapping, FEC,
Label, Attributes).
SL.5 Record label mapping for FEC with Label and Attributes has
been sent to Peer.
SL.6 Does LSR have a record of a FEC label request from Peer
marked as pending?
If not, goto SL.8.
SL.7 Delete record of pending label request for FEC from Peer.
SL.8 Return success.
SL.9 Does LSR have a label request for FEC from Peer marked as
pending?
If not, goto SL.13.
SL.10 Execute procedure Send_Notification (Peer, No Label
Resources).
SL.11 Delete record of pending label request for FEC from Peer.
SL.12 Record No Label Resources notification has been sent to
Peer.
Goto SL.14.
SL.13 Record label mapping needed for FEC and Attributes for
Peer, but no-label-resources. (See Note 1.)
SL.14 Return failure.
Note:
1. SL.13 handles the case of Downstream Unsolicited label
distribution when the LSR is unable to allocate a label for a
FEC to send to a Peer.
A.2.2. Send_Label_Request
Summary:
An LSR uses the Send_Label_Request procedure to send a request for
a label for a FEC to an LDP peer if currently permitted to do so.
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RFC 5036 LDP Specification October 2007
Parameters:
- Peer. The LDP peer to which the label request is to be sent.
- FEC. The FEC for which a label request is to be sent.
- Attributes. Attributes to be included in the label request,
e.g., Hop Count, Path Vector.
Additional Context:
- LSR. The LSR executing the procedure.
Algorithm:
SLRq.1 Has a label request for FEC previously been sent to Peer
and is it marked as outstanding?
If so, Return success. (See Note 1.)
SLRq.2 Is status record indicating it is OK to send label
requests to Peer set?
If not, goto SLRq.6
SLRq.3 Execute procedure Send_Message (Peer, Label Request, FEC,
Attributes).
SLRq.4 Record that label request for FEC has been sent to Peer
and mark it as outstanding.
SLRq.5 Return success.
SLRq.6 Postpone the label request by recording that label mapping
for FEC and Attributes from Peer is needed but that no
label resources are available.
SLRq.7 Return failure.
Note:
1. If the LSR is a non-merging LSR, it must distinguish between
attempts to send label requests for a FEC triggered by
different upstream LDP peers from duplicate requests. This
procedure will not send a duplicate label request.
Andersson, et al. Standards Track [Page 126]

RFC 5036 LDP Specification October 2007A.2.3. Send_Label_Withdraw
Summary:
An LSR uses the Send_Label_Withdraw procedure to withdraw a label
for a FEC from an LDP peer. To do this, the LSR sends a Label
Withdraw message to the peer.
Parameters:
- Peer. The LDP peer to which the label withdraw is to be sent.
- FEC. The FEC for which a label is being withdrawn.
- Label. The label being withdrawn.
Additional Context:
- LSR. The LSR executing the procedure.
Algorithm:
SWd.1 Execute procedure Send_Message (Peer, Label Withdraw, FEC,
Label).
SWd.2 Record that label withdraw for FEC has been sent to Peer
and mark it as outstanding.
A.2.4. Send_Notification
Summary:
An LSR uses the Send_Notification procedure to send an LDP peer a
Notification message.
Parameters:
- Peer. The LDP peer to which the Notification message is to be
sent.
- Status. Status code to be included in the Notification
message.
Additional Context:
None.
Andersson, et al. Standards Track [Page 127]

RFC 5036 LDP Specification October 2007
Algorithm:
SNt.1 Execute procedure Send_Message (Peer, Notification, Status)
A.2.5. Send_Message
Summary:
An LSR uses the Send_Message procedure to send an LDP peer an LDP
message.
Parameters:
- Peer. The LDP peer to which the message is to be sent.
- Message Type. The type of message to be sent.
- Additional message contents . . . .
Additional Context:
None.
Algorithm:
This procedure is the means by which an LSR sends an LDP message
of the specified type to the specified LDP peer.
A.2.6. Check_Received_Attributes
Summary:
Check the attributes received in a Label Mapping or Label Request
message. If the attributes include a Hop Count or Path Vector,
perform a Loop Detection check. If a loop is detected, cause a
Loop Detected Notification message to be sent to MsgSource.
Parameters:
- MsgSource. The LDP peer that sent the message.
- MsgType. The type of message received.
- RAttributes. The attributes in the message.
Additional Context:
- LSR Id. The unique LSR Id of this LSR.
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RFC 5036 LDP Specification October 2007
- Hop Count. The Hop Count, if any, in the received attributes.
- Path Vector. The Path Vector, if any, in the received
attributes.
Algorithm:
CRa.1 Do RAttributes include Hop Count?
If not, goto CRa.5.
CRa.2 Does Hop Count exceed Max allowable hop count?
If so, goto CRa.6.
CRa.3 Do RAttributes include Path Vector?
If not, goto CRa.5.
CRa.4 Does Path Vector include LSR Id? OR Does length of Path
Vector exceed Max allowable length?
If so, goto CRa.6
CRa.5 Return No Loop Detected.
CRa.6 Is MsgType LabelMapping?
If so, goto CRa.8. (See Note 1.)
CRa.7 Execute procedure Send_Notification (MsgSource, Loop
Detected).
CRa.8 Return Loop Detected.
CRa.9 DONE.
Note:
1. When the attributes being checked were received in a Label
Mapping message, the LSR sends the Loop Detected notification
in a Status Code TLV in a Label Release message. (See Section
"Receive Label Mapping".)
A.2.7. Prepare_Label_Request_Attributes
Summary:
This procedure is used whenever a Label Request is to be sent to a
Peer to compute the Hop Count and Path Vector, if any, to include
in the message.
Andersson, et al. Standards Track [Page 129]

RFC 5036 LDP Specification October 2007
Parameters:
- Peer. The LDP peer to which the message is to be sent.
- FEC. The FEC for which a label request is to be sent.
- RAttributes. The attributes this LSR associates with the LSP
for FEC.
- SAttributes. The attributes to be included in the Label
Request message.
Additional Context:
- LSR Id. The unique LSR Id of this LSR.
Algorithm:
PRqA.1 Is Hop Count required for this Peer? (See Note 1.) OR Do
RAttributes include a Hop Count? OR Is Loop Detection
configured on LSR?
If not, goto PRqA.14.
PRqA.2 Is LSR ingress for FEC?
If not, goto PRqA.6.
PRqA.3 Include Hop Count of 1 in SAttributes.
PRqA.4 Is Loop Detection configured on LSR? If not, goto
PRqA.14.
PRqA.5 Is LSR merge-capable?
If so, goto PRqA.14.
If not, goto PRqA.13.
PRqA.6 Do RAttributes include a Hop Count?
If not, goto PRqA.8.
PRqA.7 Increment RAttributes Hop Count and copy the resulting Hop
Count to SAttributes. (See Note 2.)
Goto PRqA.9.
PRqA.8 Include Hop Count of unknown (0) in SAttributes.
PRqA.9 Is Loop Detection configured on LSR?
If not, goto PRqA.14.
Andersson, et al. Standards Track [Page 130]

RFC 5036 LDP Specification October 2007
PRqA.10 Do RAttributes have a Path Vector?
If so, goto PRqA.12.
PRqA.11 Is LSR merge-capable?
If so, goto PRqA.14.
If not, goto PRqA.13.
PRqA.12 Add LSR Id to beginning of Path Vector from RAttributes
and copy the resulting Path Vector into SAttributes.
Goto PRqA.14.
PRqA.13 Include Path Vector of length 1 containing LSR Id in
SAttributes.
PRqA.14 DONE.
Notes:
1. The link with Peer may require that Hop Count be included in
Label Request messages; for example, see [RFC3035] and
[RFC3034].
2. For hop count arithmetic, unknown + 1 = unknown.
A.2.8. Prepare_Label_Mapping_Attributes
Summary:
This procedure is used whenever a Label Mapping is to be sent to a
Peer to compute the Hop Count and Path Vector, if any, to include
in the message.
Parameters:
- Peer. The LDP peer to which the message is to be sent.
- FEC. The FEC for which a label request is to be sent.
- RAttributes. The attributes this LSR associates with the LSP
for FEC.
- SAttributes. The attributes to be included in the Label
Mapping message.
- IsPropagating. The LSR is sending the Label Mapping message to
propagate one received from the FEC next hop.
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RFC 5036 LDP Specification October 2007
- PrevHopCount. The Hop Count, if any, this LSR associates with
the LSP for the FEC.
Additional Context:
- LSR Id. The unique LSR Id of this LSR.
Algorithm:
PMpA.1 Do the RAttributes include any unknown TLVs?
If not, goto PMpA.4.
PMpA.2 Do the settings of the U- and F-bits require forwarding of
these TLVs?
If not, goto PMpA.4.
PMpA.3 Copy the unknown TLVs in SAttributes.
PMpA.4 Is Hop Count required for this Peer? (see Note 1.) OR Do
RAttributes include a Hop Count? OR Is Loop Detection
configured on LSR?
If not, goto PMpA.24.
PMpA.5 Is LSR egress for FEC?
If not, goto PMpA.7.
PMpA.6 Include Hop Count of 1 in SAttributes. Goto PMpA.24.
PMpA.7 Do RAttributes have a Hop Count?
If not, goto PMpA.11.
PMpA.8 Is LSR a member of the edge set for an LSR domain whose
LSRs do not perform TTL decrement AND Is Peer in that
domain? (See Note 2.) If not, goto PMpA.10.
PMpA.9 Include Hop Count of 1 in SAttributes. Goto PMpA.12.
PMpA.10 Increment RAttributes Hop Count and copy the resulting Hop
Count to SAttributes. (See Note 2.) Goto PMpA.12.
PMpA.11 Include Hop Count of unknown (0) in SAttributes.
PMpA.12 Is Loop Detection configured on LSR?
If not, goto PMpA.24.
PMpA.13 Do RAttributes have a Path Vector?
If so, goto PMpA.22.
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RFC 5036 LDP Specification October 2007
PMpA.14 Is LSR propagating a received Label Mapping?
If not, goto PMpA.23.
PMpA.15 Does LSR support merging?
If not, goto PMpA.17.
PMpA.16 Has LSR previously sent a Label Mapping for FEC to Peer?
If not, goto PMpA.23.
PMpA.17 Do RAttributes include a Hop Count?
If not, goto PMpA.24.
PMpA.18 Is Hop Count in RAttributes unknown(0)?
If so, goto PMpA.23.
PMpA.19 Has LSR previously sent a Label Mapping for FEC to Peer?
If not, goto PMpA.24.
PMpA.20 Is Hop Count in RAttributes different from PrevHopCount?
If not, goto PMpA.24.
PMpA.21 Is the Hop Count in RAttributes > PrevHopCount? OR Is
PrevHopCount unknown(0)?
If not, goto PMpA.24.
PMpA.22 Add LSR Id to beginning of Path Vector from RAttributes
and copy the resulting Path Vector into SAttributes.
Goto PMpA.24.
PMpA.23 Include Path Vector of length 1 containing LSR Id in
SAttributes.
PMpA.24 DONE.
Notes:
1. The link with Peer may require that Hop Count be included in
Label Mapping messages; for example, see [RFC3035] and
[RFC3034].
2. If the LSR is at the edge of a cloud of LSRs that do not
perform TTL-decrement and it is propagating the Label Mapping
message upstream into the cloud, it sets the Hop Count to 1 so
that Hop Count across the cloud is calculated properly. This
ensures proper TTL management for packets forwarded across the
part of the LSP that passes through the cloud.
3. For hop count arithmetic, unknown + 1 = unknown.
Andersson, et al. Standards Track [Page 133]

RFC 5036 LDP Specification October 2007
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Andersson, et al. Standards Track [Page 135]